diff --git a/tests/pipeline/test_alias.py b/tests/pipeline/test_alias.py
new file mode 100644
index 0000000000..bdc68675ac
--- /dev/null
+++ b/tests/pipeline/test_alias.py
@@ -0,0 +1,65 @@
+from nose.tools import nottest
+import numpy as np
+
+from zipline.testing.predicates import assert_equal
+from zipline.pipeline import Classifier, Factor, Filter
+from zipline.utils.numpy_utils import float64_dtype, int64_dtype
+
+from .base import BasePipelineTestCase
+
+
+@nottest
+class BaseAliasTestCase(BasePipelineTestCase):
+
+    def test_alias(self):
+        f = self.Term()
+        alias = f.alias('ayy lmao')
+
+        f_values = np.random.RandomState(5).randn(5, 5)
+
+        self.check_terms(
+            terms={
+                'f_alias': alias,
+            },
+            expected={
+                'f_alias': f_values,
+            },
+            initial_workspace={f: f_values},
+            mask=self.build_mask(np.ones((5, 5))),
+        )
+
+    def test_repr(self):
+        assert_equal(
+            repr(self.Term().alias('ayy lmao')),
+            "Aliased%s(Term(...), name='ayy lmao')" % (
+                self.Term.__base__.__name__,
+            ),
+        )
+
+    def test_short_repr(self):
+        for name in ('a', 'b'):
+            assert_equal(
+                self.Term().alias(name).short_repr(),
+                name,
+            )
+
+
+class TestFactorAlias(BaseAliasTestCase):
+    class Term(Factor):
+        dtype = float64_dtype
+        inputs = ()
+        window_length = 0
+
+
+class TestFilterAlias(BaseAliasTestCase):
+    class Term(Filter):
+        inputs = ()
+        window_length = 0
+
+
+class TestClassifierAlias(BaseAliasTestCase):
+    class Term(Classifier):
+        dtype = int64_dtype
+        inputs = ()
+        window_length = 0
+        missing_value = -1
diff --git a/tests/pipeline/test_classifier.py b/tests/pipeline/test_classifier.py
index aba522259b..a716c19dca 100644
--- a/tests/pipeline/test_classifier.py
+++ b/tests/pipeline/test_classifier.py
@@ -2,10 +2,13 @@
 from operator import or_
 
 import numpy as np
+import pandas as pd
 
 from zipline.lib.labelarray import LabelArray
 from zipline.pipeline import Classifier
 from zipline.testing import parameter_space
+from zipline.testing.fixtures import ZiplineTestCase
+from zipline.testing.predicates import assert_equal
 from zipline.utils.numpy_utils import (
     categorical_dtype,
     int64_dtype,
@@ -464,3 +467,81 @@ class C(Classifier):
             "TypeError(\"unhashable type: 'dict'\",)."
         )
         self.assertEqual(errmsg, expected)
+
+
+class TestPostProcessAndToWorkSpaceValue(ZiplineTestCase):
+    def test_reversability_categorical(self):
+        class F(Classifier):
+            inputs = ()
+            window_length = 0
+            dtype = categorical_dtype
+            missing_value = '<missing>'
+
+        f = F()
+        column_data = LabelArray(
+            np.array(
+                [['a', f.missing_value],
+                 ['b', f.missing_value],
+                 ['c', 'd']],
+            ),
+            missing_value=f.missing_value,
+        )
+
+        assert_equal(
+            f.postprocess(column_data.ravel()),
+            pd.Categorical(
+                ['a', f.missing_value, 'b', f.missing_value, 'c', 'd'],
+            ),
+        )
+
+        # only include the non-missing data
+        pipeline_output = pd.Series(
+            data=['a', 'b', 'c', 'd'],
+            index=pd.MultiIndex.from_arrays([
+                [pd.Timestamp('2014-01-01'),
+                 pd.Timestamp('2014-01-02'),
+                 pd.Timestamp('2014-01-03'),
+                 pd.Timestamp('2014-01-03')],
+                [0, 0, 0, 1],
+            ]),
+            dtype='category',
+        )
+
+        assert_equal(
+            f.to_workspace_value(pipeline_output, pd.Index([0, 1])),
+            column_data,
+        )
+
+    def test_reversability_int64(self):
+        class F(Classifier):
+            inputs = ()
+            window_length = 0
+            dtype = int64_dtype
+            missing_value = -1
+
+        f = F()
+        column_data = np.array(
+            [[0, f.missing_value],
+             [1, f.missing_value],
+             [2, 3]],
+        )
+
+        assert_equal(f.postprocess(column_data.ravel()), column_data.ravel())
+
+        # only include the non-missing data
+        pipeline_output = pd.Series(
+            data=[0, 1, 2, 3],
+            index=pd.MultiIndex.from_arrays([
+                [pd.Timestamp('2014-01-01'),
+                 pd.Timestamp('2014-01-02'),
+                 pd.Timestamp('2014-01-03'),
+                 pd.Timestamp('2014-01-03')],
+                [0, 0, 0, 1],
+            ]),
+            dtype=int64_dtype,
+        )
+
+        assert_equal(
+            f.to_workspace_value(pipeline_output, pd.Index([0, 1])),
+            column_data,
+        )
diff --git a/tests/pipeline/test_engine.py b/tests/pipeline/test_engine.py
index 6a7037eb18..568b544248 100644
--- a/tests/pipeline/test_engine.py
+++ b/tests/pipeline/test_engine.py
@@ -14,6 +14,7 @@
     float32,
     float64,
     full,
+    full_like,
     log,
     nan,
     tile,
@@ -66,10 +67,12 @@
 from zipline.testing import (
     AssetID,
     AssetIDPlusDay,
+    ExplodingObject,
     check_arrays,
     make_alternating_boolean_array,
     make_cascading_boolean_array,
     OpenPrice,
+    parameter_space,
     product_upper_triangle,
 )
 from zipline.testing.fixtures import (
@@ -78,6 +81,7 @@
     WithTradingEnvironment,
     ZiplineTestCase,
 )
+from zipline.testing.predicates import assert_equal
 from zipline.utils.memoize import lazyval
 from zipline.utils.numpy_utils import bool_dtype, datetime64ns_dtype
 
@@ -163,14 +167,14 @@ def compute(self, today, assets, out, *inputs):
         out[:] = sum(inputs).sum(axis=0)
 
 
-class ConstantInputTestCase(WithTradingEnvironment, ZiplineTestCase):
+class WithConstantInputs(WithTradingEnvironment):
     asset_ids = ASSET_FINDER_EQUITY_SIDS = 1, 2, 3, 4
     START_DATE = Timestamp('2014-01-01', tz='utc')
     END_DATE = Timestamp('2014-03-01', tz='utc')
 
     @classmethod
     def init_class_fixtures(cls):
-        super(ConstantInputTestCase, cls).init_class_fixtures()
+        super(WithConstantInputs, cls).init_class_fixtures()
         cls.constants = {
             # Every day, assume every stock starts at 2, goes down to 1,
             # goes up to 4, and finishes at 3.
@@ -192,6 +196,8 @@ def init_class_fixtures(cls):
         )
         cls.assets = cls.asset_finder.retrieve_all(cls.asset_ids)
 
+
+class ConstantInputTestCase(WithConstantInputs, ZiplineTestCase):
     def test_bad_dates(self):
         loader = self.loader
         engine = SimplePipelineEngine(
@@ -1315,3 +1321,179 @@ def test_string_classifiers_produce_categoricals(self):
             columns=self.asset_finder.retrieve_all(self.asset_finder.sids),
         )
         assert_frame_equal(result.c.unstack(), expected_final_result)
+
+
+class WindowSafetyPropagationTestCase(WithSeededRandomPipelineEngine,
+                                      ZiplineTestCase):
+
+    SEEDED_RANDOM_PIPELINE_SEED = 5
+
+    def test_window_safety_propagation(self):
+        dates = self.trading_days[-30:]
+        start_date, end_date = dates[[-10, -1]]
+
+        col = TestingDataSet.float_col
+        pipe = Pipeline(
+            columns={
+                'average_of_rank_plus_one': SimpleMovingAverage(
+                    inputs=[col.latest.rank() + 1],
+                    window_length=10,
+                ),
+                'average_of_aliased_rank_plus_one': SimpleMovingAverage(
+                    inputs=[col.latest.rank().alias('some_alias') + 1],
+                    window_length=10,
+                ),
+                'average_of_rank_plus_one_aliased': SimpleMovingAverage(
+                    inputs=[(col.latest.rank() + 1).alias('some_alias')],
+                    window_length=10,
+                ),
+            }
+        )
+        results = self.run_pipeline(pipe, start_date, end_date).unstack()
+
+        expected_ranks = DataFrame(
+            self.raw_expected_values(
+                col,
+                dates[-19],
+                dates[-1],
+            ),
+            index=dates[-19:],
+            columns=self.asset_finder.retrieve_all(
+                self.ASSET_FINDER_EQUITY_SIDS,
+            )
+        ).rank(axis='columns')
+
+        # All three expressions should be equivalent and evaluate to this.
+        expected_result = (
+            (expected_ranks + 1)
+            .rolling(10)
+            .mean()
+            .dropna(how='any')
+        )
+
+        for colname in results.columns.levels[0]:
+            assert_equal(expected_result, results[colname])
+
+
+class PopulateInitialWorkspaceTestCase(WithConstantInputs, ZiplineTestCase):
+
+    @parameter_space(window_length=[3, 5], pipeline_length=[5, 10])
+    def test_populate_initial_workspace(self, window_length, pipeline_length):
+        column = USEquityPricing.low
+        base_term = column.latest
+
+        # Take a Z-Score here so that the precomputed term is window-safe.  The
+        # z-score will never actually get computed because we swap it out.
+        precomputed_term = (base_term.zscore()).alias('precomputed_term')
+
+        # A term that has `precomputed_term` as an input.
+        depends_on_precomputed_term = precomputed_term + 1
+        # A term that requires a window of `precomputed_term`.
+        depends_on_window_of_precomputed_term = SimpleMovingAverage(
+            inputs=[precomputed_term],
+            window_length=window_length,
+        )
+
+        precomputed_term_with_window = SimpleMovingAverage(
+            inputs=(column,),
+            window_length=window_length,
+        ).alias('precomputed_term_with_window')
+        depends_on_precomputed_term_with_window = (
+            precomputed_term_with_window + 1
+        )
+
+        column_value = self.constants[column]
+        precomputed_term_value = -column_value
+        precomputed_term_with_window_value = -(column_value + 1)
+
+        def populate_initial_workspace(initial_workspace,
+                                       root_mask_term,
+                                       execution_plan,
+                                       dates,
+                                       assets):
+            def shape_for_term(term):
+                ndates = len(execution_plan.mask_and_dates_for_term(
+                    term,
+                    root_mask_term,
+                    initial_workspace,
+                    dates,
+                )[1])
+                nassets = len(assets)
+                return (ndates, nassets)
+
+            ws = initial_workspace.copy()
+            ws[precomputed_term] = full(
+                shape_for_term(precomputed_term),
+                precomputed_term_value,
+                dtype=float64,
+            )
+            ws[precomputed_term_with_window] = full(
+                shape_for_term(precomputed_term_with_window),
+                precomputed_term_with_window_value,
+                dtype=float64,
+            )
+            return ws
+
+        def dispatcher(c):
+            if c is column:
+                # the base_term should never be loaded, its initial refcount
+                # should be zero
+                return ExplodingObject()
+            return self.loader
+
+        engine = SimplePipelineEngine(
+            dispatcher,
+            self.dates,
+            self.asset_finder,
+            populate_initial_workspace=populate_initial_workspace,
+        )
+
+        results = engine.run_pipeline(
+            Pipeline({
+                'precomputed_term': precomputed_term,
+                'precomputed_term_with_window': precomputed_term_with_window,
+                'depends_on_precomputed_term': depends_on_precomputed_term,
+                'depends_on_precomputed_term_with_window':
+                    depends_on_precomputed_term_with_window,
+                'depends_on_window_of_precomputed_term':
+                    depends_on_window_of_precomputed_term,
+            }),
+            self.dates[-pipeline_length],
+            self.dates[-1],
+        )
+
+        assert_equal(
+            results['precomputed_term'].values,
+            full_like(
+                results['precomputed_term'],
+                precomputed_term_value,
+            ),
+        ),
+        assert_equal(
+            results['precomputed_term_with_window'].values,
+            full_like(
+                results['precomputed_term_with_window'],
+                precomputed_term_with_window_value,
+            ),
+        ),
+        assert_equal(
+            results['depends_on_precomputed_term'].values,
+            full_like(
+                results['depends_on_precomputed_term'],
+                precomputed_term_value + 1,
+            ),
+        )
+        assert_equal(
+            results['depends_on_precomputed_term_with_window'].values,
+            full_like(
+                results['depends_on_precomputed_term_with_window'],
+                precomputed_term_with_window_value + 1,
+            ),
+        )
+        assert_equal(
+            results['depends_on_window_of_precomputed_term'].values,
+            full_like(
+                results['depends_on_window_of_precomputed_term'],
+                precomputed_term_value,
+            ),
+        )
diff --git a/tests/pipeline/test_factor.py b/tests/pipeline/test_factor.py
index f020ae8b88..2d81dc18a2 100644
--- a/tests/pipeline/test_factor.py
+++ b/tests/pipeline/test_factor.py
@@ -21,6 +21,7 @@
     where,
 )
 from numpy.random import randn, seed
+import pandas as pd
 
 from zipline.errors import UnknownRankMethod
 from zipline.lib.labelarray import LabelArray
@@ -37,6 +38,8 @@
     parameter_space,
     permute_rows,
 )
+from zipline.testing.fixtures import ZiplineTestCase
+from zipline.testing.predicates import assert_equal
 from zipline.utils.numpy_utils import (
     categorical_dtype,
     datetime64ns_dtype,
@@ -1058,3 +1061,39 @@ def test_demean_is_window_safe_if_input_is_window_safe(self):
         self.assertFalse(F().demean().window_safe)
         self.assertFalse(F(window_safe=False).demean().window_safe)
         self.assertTrue(F(window_safe=True).demean().window_safe)
+
+
+class TestPostProcessAndToWorkSpaceValue(ZiplineTestCase):
+    @parameter_space(dtype_=(float64_dtype, datetime64ns_dtype))
+    def test_reversability(self, dtype_):
+        class F(Factor):
+            inputs = ()
+            dtype = dtype_
+            window_length = 0
+
+        f = F()
+        column_data = array(
+            [[0, f.missing_value],
+             [1, f.missing_value],
+             [2, 3]],
+            dtype=dtype_,
+        )
+
+        assert_equal(f.postprocess(column_data.ravel()), column_data.ravel())
+
+        # only include the non-missing data
+        pipeline_output = pd.Series(
+            data=array([0, 1, 2, 3], dtype=dtype_),
+            index=pd.MultiIndex.from_arrays([
+                [pd.Timestamp('2014-01-01'),
+                 pd.Timestamp('2014-01-02'),
+                 pd.Timestamp('2014-01-03'),
+                 pd.Timestamp('2014-01-03')],
+                [0, 0, 0, 1],
+            ]),
+        )
+
+        assert_equal(
+            f.to_workspace_value(pipeline_output, pd.Index([0, 1])),
+            column_data,
+        )
diff --git a/tests/pipeline/test_filter.py b/tests/pipeline/test_filter.py
index 1051cee2d5..aa578a5a80 100644
--- a/tests/pipeline/test_filter.py
+++ b/tests/pipeline/test_filter.py
@@ -24,6 +24,7 @@
     sum as np_sum
 )
 from numpy.random import randn, seed as random_seed
+import pandas as pd
 
 from zipline.errors import BadPercentileBounds
 from zipline.pipeline import Filter, Factor, Pipeline
@@ -859,3 +860,38 @@ def compute(self, today, sids, out):
         assert_equal(results.odds, (sids % 2).astype(bool))
         assert_equal(results.first_five, sids < 5)
         assert_equal(results.last_three, sids >= 7)
+
+
+class TestPostProcessAndToWorkSpaceValue(ZiplineTestCase):
+    def test_reversability(self):
+        class F(Filter):
+            inputs = ()
+            window_length = 0
+            missing_value = False
+
+        f = F()
+        column_data = array(
+            [[True, f.missing_value],
+             [True, f.missing_value],
+             [True, True]],
+            dtype=bool,
+        )
+
+        assert_equal(f.postprocess(column_data.ravel()), column_data.ravel())
+
+        # only include the non-missing data
+        pipeline_output = pd.Series(
+            data=True,
+            index=pd.MultiIndex.from_arrays([
+                [pd.Timestamp('2014-01-01'),
+                 pd.Timestamp('2014-01-02'),
+                 pd.Timestamp('2014-01-03'),
+                 pd.Timestamp('2014-01-03')],
+                [0, 0, 0, 1],
+            ]),
+        )
+
+        assert_equal(
+            f.to_workspace_value(pipeline_output, pd.Index([0, 1])),
+            column_data,
+        )
diff --git a/tests/pipeline/test_term.py b/tests/pipeline/test_term.py
index c58dd4e068..39d507ca81 100644
--- a/tests/pipeline/test_term.py
+++ b/tests/pipeline/test_term.py
@@ -195,8 +195,14 @@ def check_output(graph):
             self.assertIn(SomeDataSet.bar, resolution_order)
             self.assertIn(SomeFactor(), resolution_order)
 
-            self.assertEqual(graph.node[SomeDataSet.foo]['extra_rows'], 4)
-            self.assertEqual(graph.node[SomeDataSet.bar]['extra_rows'], 4)
+            self.assertEqual(
+                graph.graph.node[SomeDataSet.foo]['extra_rows'],
+                4,
+            )
+            self.assertEqual(
+                graph.graph.node[SomeDataSet.bar]['extra_rows'],
+                4,
+            )
 
         for foobar in gen_equivalent_factors():
             check_output(self.make_execution_plan(to_dict([foobar])))
diff --git a/zipline/lib/labelarray.py b/zipline/lib/labelarray.py
index b92faf06c2..a3e8e1992e 100644
--- a/zipline/lib/labelarray.py
+++ b/zipline/lib/labelarray.py
@@ -197,6 +197,29 @@ def _from_codes_and_metadata(cls,
         ret._missing_value = missing_value
         return ret
 
+    @classmethod
+    def from_categorical(cls, categorical, missing_value=None):
+        """
+        Create a LabelArray from a pandas categorical.
+
+        Parameters
+        ----------
+        categorical : pd.Categorical
+            The categorical object to convert.
+        missing_value : bytes, unicode, or None, optional
+            The missing value to use for this LabelArray.
+
+        Returns
+        -------
+        la : LabelArray
+            The LabelArray representation of this categorical.
+        """
+        return LabelArray(
+            categorical,
+            missing_value,
+            categorical.categories,
+        )
+
     @property
     def categories(self):
         # This is a property because it should be immutable.
diff --git a/zipline/pipeline/classifiers/classifier.py b/zipline/pipeline/classifiers/classifier.py
index 73bf5a31b6..86a91a0e5b 100644
--- a/zipline/pipeline/classifiers/classifier.py
+++ b/zipline/pipeline/classifiers/classifier.py
@@ -6,6 +6,7 @@
 import re
 
 from numpy import where, isnan, nan, zeros
+import pandas as pd
 
 from zipline.lib.labelarray import LabelArray
 from zipline.lib.quantiles import quantiles
@@ -23,6 +24,7 @@
 
 from ..filters import ArrayPredicate, NotNullFilter, NullFilter, NumExprFilter
 from ..mixins import (
+    AliasedMixin,
     CustomTermMixin,
     DownsampledMixin,
     LatestMixin,
@@ -303,10 +305,42 @@ def postprocess(self, data):
             raise AssertionError("Expected a LabelArray, got %s." % type(data))
         return data.as_categorical()
 
+    def to_workspace_value(self, result, assets):
+        """
+        Called with the result of a pipeline. This needs to return an object
+        which can be put into the workspace to continue doing computations.
+
+        This is the inverse of :func:`~zipline.pipeline.term.Term.postprocess`.
+        """
+        if self.dtype == int64_dtype:
+            return super(Classifier, self).to_workspace_value(result, assets)
+
+        assert isinstance(result.values, pd.Categorical), (
+            'Expected a Categorical, got %r.' % type(result.values)
+        )
+        with_missing = pd.Series(
+            data=pd.Categorical(
+                result.values,
+                result.values.categories.union([self.missing_value]),
+            ),
+            index=result.index,
+        )
+        return LabelArray(
+            super(Classifier, self).to_workspace_value(
+                with_missing,
+                assets,
+            ),
+            self.missing_value,
+        )
+
     @classlazyval
     def _downsampled_type(self):
         return DownsampledMixin.make_downsampled_type(Classifier)
 
+    @classlazyval
+    def _aliased_type(self):
+        return AliasedMixin.make_aliased_type(Classifier)
+
 
 class Everything(Classifier):
     """
diff --git a/zipline/pipeline/engine.py b/zipline/pipeline/engine.py
index 4bb6f6b075..1d28a8349e 100644
--- a/zipline/pipeline/engine.py
+++ b/zipline/pipeline/engine.py
@@ -81,6 +81,39 @@ def run_pipeline(self, pipeline, start_date, end_date):
         )
 
 
+def default_populate_initial_workspace(initial_workspace,
+                                       root_mask_term,
+                                       execution_plan,
+                                       dates,
+                                       assets):
+    """The default implementation for ``populate_initial_workspace``. This
+    function returns the ``initial_workspace`` argument without making any
+    modifications.
+
+    Parameters
+    ----------
+    initial_workspace : dict[array-like]
+        The initial workspace before we have populated it with any cached
+        terms.
+    root_mask_term : Term
+        The root mask term, normally ``AssetExists()``. This is needed to
+        compute the dates for individual terms.
+    execution_plan : ExecutionPlan
+        The execution plan for the pipeline being run.
+    dates : pd.DatetimeIndex
+        All of the dates being requested in this pipeline run including
+        the extra dates for look back windows.
+    assets : pd.Int64Index
+        All of the assets that exist for the window being computed.
+
+    Returns
+    -------
+    populated_initial_workspace : dict[term, array-like]
+        The workspace to begin computations with.
+    """
+    return initial_workspace
+
+
 class SimplePipelineEngine(object):
     """
     PipelineEngine class that computes each term independently.
@@ -96,6 +129,15 @@ class SimplePipelineEngine(object):
     asset_finder : zipline.assets.AssetFinder
         An AssetFinder instance.  We depend on the AssetFinder to determine
         which assets are in the top-level universe at any point in time.
+    populate_initial_workspace : callable, optional
+        A function which will be used to populate the initial workspace when
+        computing a pipeline. See
+        :func:`zipline.pipeline.engine.default_populate_initial_workspace`
+        for more info.
+
+    See Also
+    --------
+    :func:`zipline.pipeline.engine.default_populate_initial_workspace`
     """
     __slots__ = (
         '_get_loader',
@@ -103,10 +145,15 @@ class SimplePipelineEngine(object):
         '_finder',
         '_root_mask_term',
         '_root_mask_dates_term',
+        '_populate_initial_workspace',
         '__weakref__',
     )
 
-    def __init__(self, get_loader, calendar, asset_finder):
+    def __init__(self,
+                 get_loader,
+                 calendar,
+                 asset_finder,
+                 populate_initial_workspace=None):
         self._get_loader = get_loader
         self._calendar = calendar
         self._finder = asset_finder
@@ -114,6 +161,10 @@ def __init__(self, get_loader, calendar, asset_finder):
         self._root_mask_term = AssetExists()
         self._root_mask_dates_term = InputDates()
 
+        self._populate_initial_workspace = (
+            populate_initial_workspace or default_populate_initial_workspace
+        )
+
     def run_pipeline(self, pipeline, start_date, end_date):
         """
         Compute a pipeline.
@@ -179,14 +230,22 @@ def run_pipeline(self, pipeline, start_date, end_date):
         root_mask = self._compute_root_mask(start_date, end_date, extra_rows)
         dates, assets, root_mask_values = explode(root_mask)
 
+        initial_workspace = self._populate_initial_workspace(
+            {
+                self._root_mask_term: root_mask_values,
+                self._root_mask_dates_term: as_column(dates.values)
+            },
+            self._root_mask_term,
+            graph,
+            dates,
+            assets,
+        )
+
         results = self.compute_chunk(
             graph,
             dates,
             assets,
-            initial_workspace={
-                self._root_mask_term: root_mask_values,
-                self._root_mask_dates_term: as_column(dates.values)
-            },
+            initial_workspace,
         )
 
         return self._to_narrow(
@@ -255,21 +314,6 @@ def _compute_root_mask(self, start_date, end_date, extra_rows):
         assert shape[0] * shape[1] != 0, 'root mask cannot be empty'
         return ret
 
-    def _mask_and_dates_for_term(self, term, workspace, graph, all_dates):
-        """
-        Load mask and mask row labels for term.
-        """
-        mask = term.mask
-        mask_offset = graph.extra_rows[mask] - graph.extra_rows[term]
-
-        # This offset is computed against _root_mask_term because that is what
-        # determines the shape of the top-level dates array.
-        dates_offset = (
-            graph.extra_rows[self._root_mask_term] - graph.extra_rows[term]
-        )
-
-        return workspace[mask][mask_offset:], all_dates[dates_offset:]
-
     @staticmethod
     def _inputs_for_term(term, workspace, graph):
         """
@@ -346,7 +390,7 @@ def compute_chunk(self, graph, dates, assets, initial_workspace):
 
         refcounts = graph.initial_refcounts(workspace)
 
-        for term in graph.ordered():
+        for term in graph.execution_order(refcounts):
             # `term` may have been supplied in `initial_workspace`, and in the
             # future we may pre-compute loadable terms coming from the same
             # dataset.  In either case, we will already have an entry for this
@@ -356,8 +400,11 @@ def compute_chunk(self, graph, dates, assets, initial_workspace):
 
             # Asset labels are always the same, but date labels vary by how
             # many extra rows are needed.
-            mask, mask_dates = self._mask_and_dates_for_term(
-                term, workspace, graph, dates
+            mask, mask_dates = graph.mask_and_dates_for_term(
+                term,
+                self._root_mask_term,
+                workspace,
+                dates,
             )
 
             if isinstance(term, LoadableTerm):
diff --git a/zipline/pipeline/expression.py b/zipline/pipeline/expression.py
index d99ab341db..941f058e47 100644
--- a/zipline/pipeline/expression.py
+++ b/zipline/pipeline/expression.py
@@ -187,6 +187,7 @@ def __new__(cls, expr, binds, dtype):
             inputs=binds,
             expr=expr,
             dtype=dtype,
+            window_safe=all(t.window_safe for t in binds),
         )
 
     def _init(self, expr, *args, **kwargs):
diff --git a/zipline/pipeline/factors/factor.py b/zipline/pipeline/factors/factor.py
index 3566abdc94..793282d10a 100644
--- a/zipline/pipeline/factors/factor.py
+++ b/zipline/pipeline/factors/factor.py
@@ -32,6 +32,7 @@
     NullFilter,
 )
 from zipline.pipeline.mixins import (
+    AliasedMixin,
     CustomTermMixin,
     DownsampledMixin,
     LatestMixin,
@@ -1078,6 +1079,10 @@ def isfinite(self):
     def _downsampled_type(self):
         return DownsampledMixin.make_downsampled_type(Factor)
 
+    @classlazyval
+    def _aliased_type(self):
+        return AliasedMixin.make_aliased_type(Factor)
+
 
 class NumExprFactor(NumericalExpression, Factor):
     """
diff --git a/zipline/pipeline/filters/filter.py b/zipline/pipeline/filters/filter.py
index 89cd790a33..7f17588bf1 100644
--- a/zipline/pipeline/filters/filter.py
+++ b/zipline/pipeline/filters/filter.py
@@ -24,6 +24,7 @@
     NumericalExpression,
 )
 from zipline.pipeline.mixins import (
+    AliasedMixin,
     CustomTermMixin,
     DownsampledMixin,
     LatestMixin,
@@ -207,6 +208,10 @@ def _validate(self):
     def _downsampled_type(self):
         return DownsampledMixin.make_downsampled_type(Filter)
 
+    @classlazyval
+    def _aliased_type(self):
+        return AliasedMixin.make_aliased_type(Filter)
+
 
 class NumExprFilter(NumericalExpression, Filter):
     """
diff --git a/zipline/pipeline/graph.py b/zipline/pipeline/graph.py
index 667c3be9b0..89107f0394 100644
--- a/zipline/pipeline/graph.py
+++ b/zipline/pipeline/graph.py
@@ -16,7 +16,7 @@ class CyclicDependency(Exception):
     pass
 
 
-class TermGraph(DiGraph):
+class TermGraph(object):
     """
     An abstract representation of Pipeline Term dependencies.
 
@@ -44,7 +44,7 @@ class TermGraph(DiGraph):
     ExecutionPlan
     """
     def __init__(self, terms):
-        super(TermGraph, self).__init__()
+        self.graph = DiGraph()
 
         self._frozen = False
         parents = set()
@@ -54,7 +54,6 @@ def __init__(self, terms):
             assert not parents
 
         self._outputs = terms
-        self._ordered = topological_sort(self)
 
         # Mark that no more terms should be added to the graph.
         self._frozen = True
@@ -79,11 +78,11 @@ def _add_to_graph(self, term, parents):
 
         parents.add(term)
 
-        self.add_node(term)
+        self.graph.add_node(term)
 
         for dependency in term.dependencies:
             self._add_to_graph(dependency, parents)
-            self.add_edge(dependency, term)
+            self.graph.add_edge(dependency, term)
 
         parents.remove(term)
 
@@ -94,15 +93,25 @@ def outputs(self):
         """
         return self._outputs
 
-    def ordered(self):
+    def execution_order(self, refcounts):
         """
-        Return a topologically-sorted iterator over the terms in `self`.
+        Return a topologically-sorted iterator over the terms in ``self`` which
+        need to be computed.
         """
-        return iter(self._ordered)
+        return iter(topological_sort(
+            self.graph.subgraph(
+                {term for term, refcount in refcounts.items() if refcount > 0},
+            ),
+        ))
+
+    def ordered(self):
+        return iter(topological_sort(self.graph))
 
     @lazyval
     def loadable_terms(self):
-        return tuple(term for term in self if isinstance(term, LoadableTerm))
+        return tuple(
+            term for term in self.graph if isinstance(term, LoadableTerm)
+        )
 
     @lazyval
     def jpeg(self):
@@ -132,15 +141,34 @@ def initial_refcounts(self, initial_terms):
         nodes get one extra reference to ensure that they're still in the graph
         at the end of execution.
         """
-        refcounts = self.out_degree()
+        refcounts = self.graph.out_degree()
         for t in self.outputs.values():
             refcounts[t] += 1
 
         for t in initial_terms:
-            self.decref_dependencies(t, refcounts)
+            self._decref_depencies_recursive(t, refcounts, set())
 
         return refcounts
 
+    def _decref_depencies_recursive(self, term, refcounts, garbage):
+        """
+        Decrement terms recursively.
+
+        Notes
+        -----
+        This should only be used to build the initial workspace, after that we
+        should use:
+        :meth:`~zipline.pipeline.graph.TermGraph.decref_dependencies`
+        """
+        # Edges are tuple of (from, to).
+        for parent, _ in self.graph.in_edges([term]):
+            refcounts[parent] -= 1
+            # No one else depends on this term. Remove it from the
+            # workspace to conserve memory.
+            if refcounts[parent] == 0:
+                garbage.add(parent)
+                self._decref_depencies_recursive(parent, refcounts, garbage)
+
     def decref_dependencies(self, term, refcounts):
         """
         Decrement in-edges for ``term`` after computation.
@@ -159,7 +187,7 @@ def decref_dependencies(self, term, refcounts):
         """
         garbage = set()
         # Edges are tuple of (from, to).
-        for parent, _ in self.in_edges([term]):
+        for parent, _ in self.graph.in_edges([term]):
             refcounts[parent] -= 1
             # No one else depends on this term. Remove it from the
             # workspace to conserve memory.
@@ -326,7 +354,7 @@ def offset(self):
             # How much bigger is the array for ``dep`` compared to ``term``?
             # How much of that difference did I ask for.
             (term, dep): (extra[dep] - extra[term]) - requested_extra_rows
-            for term in self
+            for term in self.graph
             for dep, requested_extra_rows in term.dependencies.items()
         }
 
@@ -366,12 +394,49 @@ def extra_rows(self):
         """
         return {
             term: attrs['extra_rows']
-            for term, attrs in iteritems(self.node)
+            for term, attrs in iteritems(self.graph.node)
         }
 
     def _ensure_extra_rows(self, term, N):
         """
         Ensure that we're going to compute at least N extra rows of `term`.
         """
-        attrs = self.node[term]
+        attrs = self.graph.node[term]
         attrs['extra_rows'] = max(N, attrs.get('extra_rows', 0))
+
+    def mask_and_dates_for_term(self,
+                                term,
+                                root_mask_term,
+                                workspace,
+                                all_dates):
+        """
+        Load mask and mask row labels for term.
+
+        Parameters
+        ----------
+        term : Term
+            The term to load the mask and labels for.
+        root_mask_term : Term
+            The term that represents the root asset exists mask.
+        workspace : dict[Term, any]
+            The values that have been computed for each term.
+        all_dates : pd.DatetimeIndex
+            All of the dates that are being computed for in the pipeline.
+
+        Returns
+        -------
+        mask : np.ndarray
+            The correct mask for this term.
+        dates : np.ndarray
+            The slice of dates for this term.
+        """
+        mask = term.mask
+        mask_offset = self.extra_rows[mask] - self.extra_rows[term]
+
+        # This offset is computed against root_mask_term because that is what
+        # determines the shape of the top-level dates array.
+        dates_offset = (
+            self.extra_rows[root_mask_term] - self.extra_rows[term]
+        )
+
+        return workspace[mask][mask_offset:], all_dates[dates_offset:]
diff --git a/zipline/pipeline/mixins.py b/zipline/pipeline/mixins.py
index a18251011b..6271abcc34 100644
--- a/zipline/pipeline/mixins.py
+++ b/zipline/pipeline/mixins.py
@@ -20,6 +20,7 @@
 from zipline.utils.input_validation import expect_types
 from zipline.utils.sharedoc import (
     format_docstring,
+    PIPELINE_ALIAS_NAME_DOC,
     PIPELINE_DOWNSAMPLING_FREQUENCY_DOC,
 )
 from zipline.utils.pandas_utils import nearest_unequal_elements
@@ -240,6 +241,77 @@ def _validate(self):
             )
 
 
+class AliasedMixin(SingleInputMixin):
+    """
+    Mixin for aliased terms.
+    """
+    def __new__(cls, term, name):
+        return super(AliasedMixin, cls).__new__(
+            cls,
+            inputs=(term,),
+            outputs=term.outputs,
+            window_length=0,
+            name=name,
+            dtype=term.dtype,
+            missing_value=term.missing_value,
+            ndim=term.ndim,
+            window_safe=term.window_safe,
+        )
+
+    def _init(self, name, *args, **kwargs):
+        self.name = name
+        return super(AliasedMixin, self)._init(*args, **kwargs)
+
+    @classmethod
+    def _static_identity(cls, name, *args, **kwargs):
+        return (
+            super(AliasedMixin, cls)._static_identity(*args, **kwargs),
+            name,
+        )
+
+    def _compute(self, inputs, dates, assets, mask):
+        return inputs[0]
+
+    def __repr__(self):
+        return '{type}({inner_type}(...), name={name!r})'.format(
+            type=type(self).__name__,
+            inner_type=type(self.inputs[0]).__name__,
+            name=self.name,
+        )
+
+    def short_repr(self):
+        return self.name
+
+    @classmethod
+    def make_aliased_type(cls, other_base):
+        """
+        Factory for making Aliased{Filter,Factor,Classifier}.
+        """
+        docstring = dedent(
+            """
+            A {t} that names another {t}.
+
+            Parameters
+            ----------
+            term : {t}
+            {{name}}
+            """
+        ).format(t=other_base.__name__)
+
+        doc = format_docstring(
+            owner_name=other_base.__name__,
+            docstring=docstring,
+            formatters={'name': PIPELINE_ALIAS_NAME_DOC},
+        )
+
+        return type(
+            'Aliased' + other_base.__name__,
+            (cls, other_base),
+            {'__doc__': doc,
+             '__module__': other_base.__module__},
+        )
+
+
 class DownsampledMixin(StandardOutputs):
     """
     Mixin for behavior shared by Downsampled{Factor,Filter,Classifier}
diff --git a/zipline/pipeline/term.py b/zipline/pipeline/term.py
index 6cecd1475e..284f536dc4 100644
--- a/zipline/pipeline/term.py
+++ b/zipline/pipeline/term.py
@@ -39,6 +39,7 @@
 )
 from zipline.utils.sharedoc import (
     templated_docstring,
+    PIPELINE_ALIAS_NAME_DOC,
     PIPELINE_DOWNSAMPLING_FREQUENCY_DOC,
 )
 
@@ -590,7 +591,32 @@ def postprocess(self, data):
         """
         return data
 
-    def _downsampled_type(self):
+    def to_workspace_value(self, result, assets):
+        """
+        Called with a column of the result of a pipeline. This needs to put
+        the data into a format that can be used in a workspace to continue
+        doing computations.
+
+        Parameters
+        ----------
+        result : pd.Series
+            A multiindexed series with (dates, assets) whose values are the
+            results of running this pipeline term over the dates.
+        assets : pd.Index
+            All of the assets being requested. This allows us to correctly
+            shape the workspace value.
+
+        Returns
+        -------
+        workspace_value : array-like
+            An array like value that the engine can consume.
+        """
+        return result.unstack().fillna(self.missing_value).reindex(
+            columns=assets,
+            fill_value=self.missing_value,
+        ).values
+
+    def _downsampled_type(self, *args, **kwargs):
         """
         The expression type to return from self.downsample().
         """
@@ -611,6 +637,35 @@ def downsample(self, frequency):
         """
         return self._downsampled_type(term=self, frequency=frequency)
 
+    def _aliased_type(self, *args, **kwargs):
+        """
+        The expression type to return from self.alias().
+        """
+        raise NotImplementedError(
+            "alias is not yet implemented "
+            "for instances of %s." % type(self).__name__
+        )
+
+    @templated_docstring(name=PIPELINE_ALIAS_NAME_DOC)
+    def alias(self, name):
+        """
+        Make a term from ``self`` that names the expression.
+
+        Parameters
+        ----------
+        {name}
+
+        Returns
+        -------
+        aliased : Aliased
+            ``self`` with a name.
+
+        Notes
+        -----
+        This is useful for giving a name to a numerical or boolean expression.
+        """
+        return self._aliased_type(term=self, name=name)
+
     def __repr__(self):
         return (
             "{type}({inputs}, window_length={window_length})"
diff --git a/zipline/pipeline/visualize.py b/zipline/pipeline/visualize.py
index f0356b4375..fdc34b70af 100644
--- a/zipline/pipeline/visualize.py
+++ b/zipline/pipeline/visualize.py
@@ -115,13 +115,14 @@ def _render(g, out, format_, include_asset_exists=False):
                 add_term_node(f, term)
 
         # Write intermediate results.
-        for term in filter_nodes(include_asset_exists, topological_sort(g)):
+        for term in filter_nodes(include_asset_exists,
+                                 topological_sort(g.graph)):
             if term in in_nodes or term in out_nodes:
                 continue
             add_term_node(f, term)
 
         # Write edges
-        for source, dest in g.edges():
+        for source, dest in g.graph.edges():
             if source is AssetExists() and not include_asset_exists:
                 continue
             add_edge(f, id(source), id(dest))
diff --git a/zipline/testing/predicates.py b/zipline/testing/predicates.py
index da62bdb639..cf4f042e5e 100644
--- a/zipline/testing/predicates.py
+++ b/zipline/testing/predicates.py
@@ -39,6 +39,7 @@
     assert_frame_equal,
     assert_panel_equal,
     assert_series_equal,
+    assert_index_equal,
 )
 from six import iteritems, viewkeys, PY2
 from toolz import dissoc, keyfilter
@@ -47,6 +48,7 @@
 from zipline.testing.core import ensure_doctest
 from zipline.dispatch import dispatch
 from zipline.lib.adjustment import Adjustment
+from zipline.lib.labelarray import LabelArray
 from zipline.utils.functional import dzip_exact, instance
 from zipline.utils.math_utils import tolerant_equals
 
@@ -425,7 +427,23 @@ def assert_array_equal(result,
         raise AssertionError('\n'.join((str(e), _fmt_path(path))))
 
 
-def _register_assert_ndframe_equal(type_, assert_eq):
+@assert_equal.register(LabelArray, LabelArray)
+def assert_labelarray_equal(result, expected, path=(), **kwargs):
+    assert_equal(
+        result.categories,
+        expected.categories,
+        path=path + ('.categories',),
+        **kwargs
+    )
+    assert_equal(
+        result.as_int_array(),
+        expected.as_int_array(),
+        path=path + ('.as_int_array()',),
+        **kwargs
+    )
+
+
+def _register_assert_equal_wrapper(type_, assert_eq):
     """Register a new check for an ndframe object.
 
     Parameters
@@ -456,18 +474,40 @@ def assert_ndframe_equal(result, expected, path=(), msg='', **kwargs):
     return assert_ndframe_equal
 
 
-assert_frame_equal = _register_assert_ndframe_equal(
+assert_frame_equal = _register_assert_equal_wrapper(
     pd.DataFrame,
     assert_frame_equal,
 )
-assert_panel_equal = _register_assert_ndframe_equal(
+assert_panel_equal = _register_assert_equal_wrapper(
     pd.Panel,
     assert_panel_equal,
 )
-assert_series_equal = _register_assert_ndframe_equal(
+assert_series_equal = _register_assert_equal_wrapper(
     pd.Series,
     assert_series_equal,
 )
+assert_index_equal = _register_assert_equal_wrapper(
+    pd.Index,
+    assert_index_equal,
+)
+
+
+@assert_equal.register(pd.Categorical, pd.Categorical)
+def assert_categorical_equal(result, expected, path=(), msg='', **kwargs):
+    assert_equal(
+        result.categories,
+        expected.categories,
+        path=path + ('.categories',),
+        msg=msg,
+        **kwargs
+    )
+    assert_equal(
+        result.codes,
+        expected.codes,
+        path=path + ('.codes',),
+        msg=msg,
+        **kwargs
+    )
 
 
 @assert_equal.register(Adjustment, Adjustment)
diff --git a/zipline/utils/sharedoc.py b/zipline/utils/sharedoc.py
index 9669366c82..d0d7248c2d 100644
--- a/zipline/utils/sharedoc.py
+++ b/zipline/utils/sharedoc.py
@@ -18,6 +18,13 @@
     """
 )
 
+PIPELINE_ALIAS_NAME_DOC = dedent(
+    """\
+    name : str
+        The name to alias this term as.
+    """,
+)
+
 
 def pad_lines_after_first(prefix, s):
     """Apply a prefix to each line in s after the first."""