diff --git a/mlens/parallel/_base_functions.py b/mlens/parallel/_base_functions.py
index b0f77725..3f23e5c1 100644
--- a/mlens/parallel/_base_functions.py
+++ b/mlens/parallel/_base_functions.py
@@ -79,7 +79,7 @@ def replace(source_files):
 
 
 def mold_objects(learners, transformers):
-    """Utility for enforcing list"""
+    """Utility for enforcing compatible setup"""
     out = []
     for objects in [learners, transformers]:
         if objects:
diff --git a/mlens/parallel/backend.py b/mlens/parallel/backend.py
index 01731709..c19b4696 100644
--- a/mlens/parallel/backend.py
+++ b/mlens/parallel/backend.py
@@ -34,6 +34,11 @@
 
 
 ###############################################################################
+def _dtype(a, b=None):
+    """Utility for getting a dtype"""
+    return getattr(a, 'dtype', getattr(b, 'dtype', None))
+
+
 def dump_array(array, name, path):
     """Dump array for memmapping."""
     # First check if the array is on file
@@ -538,12 +543,12 @@ def process(self, caller, out, **kwargs):
                 self._partial_process(task, parallel, **kwargs)
 
                 if task.name in return_names:
-                    out.append(self.get_preds(getattr(task, 'dtype', None)))
+                    out.append(self.get_preds(dtype=_dtype(task)))
 
                 self.job.update()
 
         if return_final:
-            out = self.get_preds(dtype=getattr(task, 'dtype', None))
+            out = self.get_preds(dtype=_dtype(task))
         return out
 
     def _partial_process(self, task, parallel, **kwargs):
@@ -582,19 +587,18 @@ def _gen_prediction_array(self, task, job, threading):
         """Generate prediction array either in-memory or persist to disk."""
         shape = task.shape(job)
         if threading:
-            self.job.predict_out = np.empty(
-                shape, dtype=getattr(task, 'dtype', config.get_dtype()))
+            self.job.predict_out = np.empty(shape, dtype=_dtype(task))
         else:
             f = os.path.join(self.job.dir, '%s_out_array.mmap' % task.name)
             try:
                 self.job.predict_out = np.memmap(
-                    filename=f, dtype=task.dtype, mode='w+', shape=shape)
+                    filename=f, dtype=_dtype(task), mode='w+', shape=shape)
             except Exception as exc:
-                raise OSError("Cannot create prediction matrix of shape ("
-                              "%i, %i), size %i MBs, for %s.\n Details:\n%r" %
-                              (shape[0], shape[1],
-                               8 * shape[0] * shape[1] / (1024 ** 2),
-                               task.name, exc))
+                raise OSError(
+                    "Cannot create prediction matrix of shape ("
+                    "%i, %i), size %i MBs, for %s.\n Details:\n%r" %
+                    (shape[0], shape[1], 8 * shape[0] * shape[1] / (1024 ** 2),
+                     task.name, exc))
 
     def get_preds(self, dtype=None, order='C'):
         """Return prediction matrix.
diff --git a/mlens/parallel/base.py b/mlens/parallel/base.py
index 549d8d92..cf97020c 100644
--- a/mlens/parallel/base.py
+++ b/mlens/parallel/base.py
@@ -355,8 +355,7 @@ def get_params(self, deep=True):
     @property
     def __fitted__(self):
         """Fitted status"""
-        if not self.stack:
-            # all on an empty list yields True
+        if not self.stack or not self._check_static_params():
             return False
         return all([g.__fitted__ for g in self.stack])
 
@@ -376,6 +375,6 @@ def verbose(self):
     @verbose.setter
     def verbose(self, verbose):
         """Set verbosity"""
+        self._verbose = verbose
         for g in self.stack:
-            for obj in g:
-                obj.verbose = verbose
+            g.verbose = verbose
diff --git a/mlens/parallel/handles.py b/mlens/parallel/handles.py
index 16fb6cc0..403b244c 100644
--- a/mlens/parallel/handles.py
+++ b/mlens/parallel/handles.py
@@ -6,17 +6,18 @@
 
 Handles for mlens.parallel objects.
 """
+from .base import BaseEstimator
 from .learner import Learner, Transformer
 from ._base_functions import mold_objects, transform
 from ..utils import format_name, check_instances
 from ..utils.formatting import _check_instances
-from ..externals.sklearn.base import BaseEstimator, clone
+from ..externals.sklearn.base import clone, BaseEstimator as _BaseEstimator
 
 GLOBAL_GROUP_NAMES = list()
 GLOBAL_PIPELINE_NAMES = list()
 
 
-class Pipeline(BaseEstimator):
+class Pipeline(_BaseEstimator):
 
     """Transformer pipeline
 
@@ -36,7 +37,7 @@ class Pipeline(BaseEstimator):
         name of pipeline.
 
     return_y: bool (default=False)
-        If True, both X and y will be returned in a transform call.
+        If True, both X and y will be returned in a :method:`transform` call.
     """
 
     def __init__(self, pipeline, name=None, return_y=False):
@@ -106,8 +107,8 @@ def transform(self, X, y=None):
         """Transform pipeline.
 
         Note that the :class:`Pipeline` accepts both X and y arguments, and
-        can return both X and y, depending on the transformers. The
-        pipeline itself does no checks on the input.
+        can return both X and y, depending on the transformers.
+        Pipeline itself does not checks the input.
 
         Parameters
         ----------
@@ -123,7 +124,7 @@ def transform(self, X, y=None):
             Preprocessed input data
 
         y: array-like of shape [n_samples, ], optional
-            Preprocessed targets
+            Original or preprocessed targets, depending on the transformers.
         """
         return self._run(False, True, X, y)
 
@@ -172,25 +173,38 @@ class Group(BaseEstimator):
     a set of transformers that all share the same cross-validation strategy.
     All instances will share *the same* indexer. Allows cloning.
 
+    Acceptable caller to :class:`ParallelProcessing`.
+
     Parameters
     ----------
-    indexer: inst
+    indexer: inst, optional
         A :mod:`mlens.index` indexer to build learner and transformers on.
+        If not passed, the first indexer of the learners will be enforced
+        on all instances.
 
     learners: list, inst, optional
-        :class:`Learner` instance(s) to build on indexer.
+        :class:`Learner` instance(s) attached to indexer. Note that
+        :class:`Group` overrides previous ``indexer`` parameter settings.
 
     transformers: list, inst, optional
-        :class:`Transformer` instance(s) to build on indexer.
+        :class:`Transformer` instance(s) attached to indexer. Note that
+        :class:`Group` overrides previous ``indexer`` parameter settings.
 
     name: str, optional
         name of group
+
+    **kwargs: optional
+        Optional keyword arguments to :class:`BaseParallel` backend.
     """
 
-    def __init__(self, indexer, learners=None, transformers=None, name=None):
+    def __init__(self, indexer=None, learners=None, transformers=None,
+                 name=None, **kwargs):
+        name = format_name(name, 'group', GLOBAL_GROUP_NAMES)
+        super(Group, self).__init__(name=name, **kwargs)
 
-        self.name = format_name(name, 'group', GLOBAL_GROUP_NAMES)
         learners, transformers = mold_objects(learners, transformers)
+        if not indexer:
+            indexer = learners[0].indexer
 
         # Enforce common indexer
         self.indexer = indexer
@@ -200,15 +214,28 @@ def __init__(self, indexer, learners=None, transformers=None, name=None):
         self.learners = learners
         self.transformers = transformers
 
+        self.__static__.extend(['indexer', 'learners', 'transformers'])
+
     def __iter__(self):
+        # We update optional backend kwargs that might have been passed
+        # to ensure these are passed to the instances
+        backend_kwargs = {
+            param: getattr(self, param)
+            for param in ['dtype', 'verbose', 'raise_on_exception']
+            if hasattr(self, param)
+        }
         for tr in self.transformers:
+            tr.set_params(**backend_kwargs)
             yield tr
         for lr in self.learners:
+            lr.set_params(**backend_kwargs)
             yield lr
 
     @property
     def __fitted__(self):
         """Fitted status"""
+        if not self._check_static_params():
+            return False
         return all([o.__fitted__ for o in self.learners + self.transformers])
 
     def get_params(self, deep=True):
diff --git a/mlens/parallel/learner.py b/mlens/parallel/learner.py
index 79c687be..be18cd59 100644
--- a/mlens/parallel/learner.py
+++ b/mlens/parallel/learner.py
@@ -412,12 +412,15 @@ class _BaseEstimator(OutputMixin, IndexMixin, BaseEstimator):
 
     """Base estimator class
 
-    Common API for estimation objects.
+    Common API for job generators. A class that inherits the base
+    need to set a ``__subtype__`` in the constructor. The sub-type should be
+    the class that runs estimations and must implement a ``__call__``,
+    ``fit``, ``transform`` and ``predict`` method.
     """
 
     __meta_class__ = ABCMeta
 
-    # Reset subtype class attribute in any class that inherits
+    # Reset subtype class attribute in any class that inherits the base
     __subtype__ = None
 
     def __init__(self, name, estimator, indexer=None, verbose=False, **kwargs):
@@ -438,12 +441,14 @@ def __init__(self, name, estimator, indexer=None, verbose=False, **kwargs):
         if self.indexer:
             self.set_indexer(self.indexer)
 
-        self.cache_name = self.name
         self.estimator = estimator
         self.verbose = verbose
+        self.cache_name = None
         self.output_columns = None
         self.feature_span = None
 
+        self.__static__.extend(['estimator', 'name', 'indexer'])
+
     def __iter__(self):
         yield self
 
@@ -529,6 +534,15 @@ def gen_fit(self, X, y, P=None):
             output array to populate. Must be writeable. Only pass if
             predictions are desired.
         """
+        # We use a derived cache_name during estimation: if the name of the
+        # instance or the name of the preprocessing dependency changes, this
+        # allows us to pick up on that.
+        if hasattr(self, 'preprocess'):
+            self.cache_name = '%s.%s' % (
+                self.preprocess, self.name) if self.preprocess else self.name
+        else:
+            self.cache_name = self.name
+
         if self.__subtype__ is None:
             raise ParallelProcessingError(
                 "Class incorrectly constructed. Need to set class attribute "
@@ -826,11 +840,6 @@ def __init__(self, estimator, indexer=None, name=None, preprocess=None,
         # Protect preprocess against later changes
         self.__static__.append('preprocess')
 
-    def gen_fit(self, X, y, P=None):
-        self.cache_name = '%s.%s' % (
-            self.preprocess, self.name) if self.preprocess else self.name
-        return super(Learner, self).gen_fit(X, y, P)
-
     @property
     def scorer(self):
         """Copy of scorer"""
@@ -903,8 +912,9 @@ class EvalTransformer(Transformer):
     See :class:`Transformer` for more details.
     """
 
-    def __init__(self, *args, **kwargs):
-        super(EvalTransformer, self).__init__(*args, **kwargs)
+    def __init__(self, estimator, indexer=None, name=None, **kwargs):
+        super(EvalTransformer, self).__init__(
+            estimator, indexer=indexer, name=name, **kwargs)
         self.output_columns = {0: 0}  # For compatibility with SubTransformer
         self.__only_all__ = False
         self.__only_sub__ = True
diff --git a/mlens/parallel/tests/test_d_wrapper.py b/mlens/parallel/tests/test_d_wrapper.py
index 01865bea..c517dfef 100644
--- a/mlens/parallel/tests/test_d_wrapper.py
+++ b/mlens/parallel/tests/test_d_wrapper.py
@@ -4,7 +4,7 @@
 """
 import numpy as np
 from mlens.testing import Data, EstimatorContainer
-from mlens.parallel import Learner, Transformer, Pipeline, run as _run
+from mlens.parallel import Group, Learner, Transformer, Pipeline, run as _run
 from mlens.utils.dummy import OLS, Scale
 from mlens.externals.sklearn.base import clone
 
@@ -23,8 +23,9 @@
 def scorer(p, y): return np.mean(p - y)
 
 
-data = Data('stack', True, False)
+data = Data('stack', False, True, True)
 X, y = data.get_data((25, 4), 3)
+(F, wf), (P, wp) = data.ground_truth(X, y,)
 
 
 if SKLEARN:
@@ -165,42 +166,28 @@ def test_tr_subsemble():
 
 def test_run_fit():
     """[Parallel | Wrapper] test fit with auxiliary"""
-    tr = Transformer(Pipeline(Scale(), return_y=True),
-                     name='pr', indexer=data.indexer)
-    lr = Learner(OLS(),
-                 indexer=data.indexer, name='lr', preprocess='pr',
-                 scorer=scorer)
+    lr, tr = EstimatorContainer().get_learner('stack', False, True)
+    group = Group(learners=lr, transformers=tr, dtype=np.float64)
 
-    out = _run([tr, lr], 'fit', X, y)
-    assert out is None
+    A = _run(group, 'fit', X, y, return_preds=True)
+    np.testing.assert_array_equal(A, F)
 
 
 def test_run_transform():
     """[Parallel | Wrapper] test transform with auxiliary"""
-    tr = Transformer(Pipeline(Scale(), return_y=True), indexer=data.indexer)
-    lr = Learner(OLS(), indexer=data.indexer, scorer=scorer)
+    lr, tr = EstimatorContainer().get_learner('stack', False, True)
+    group = Group(learners=lr, transformers=tr, dtype=np.float64)
 
-    # Indirect stack
-    X_ = _run(tr, 'fit', X, y, return_preds=True)
-    A = _run(lr, 'fit', X_, y, return_preds=True)
-
-    # Direct stack
-    B = _run([tr, lr], 'transform', X, y, map=False)
-
-    np.testing.assert_array_equal(A, B)
+    _run(group, 'fit', X, y)
+    A = _run(group, 'transform', X)
+    np.testing.assert_array_equal(A, F)
 
 
 def test_run_predict():
     """[Parallel | Wrapper] test predict with auxiliary"""
-    tr = Transformer(Pipeline(Scale(), return_y=True),
-                     name='pr', indexer=data.indexer)
-    lr = Learner(OLS(), indexer=data.indexer, name='lr', scorer=scorer)
-
-    _run([tr, lr], 'fit', X, y, map=False)
-    B = _run([tr, lr], 'predict', X, y, map=False)
-
-    X1 = _run(tr, 'transform', X)
-    X2 = _run(tr, 'predict', X)
-    A = OLS().fit(X1, y).predict(X2)
+    lr, tr = EstimatorContainer().get_learner('stack', False, True)
+    group = Group(learners=lr, transformers=tr, dtype=np.float64)
 
-    np.testing.assert_array_equal(A.astype(np.float32), B.ravel())
+    _run(group, 'fit', X, y)
+    A = _run(group, 'predict', X)
+    np.testing.assert_array_equal(A, P)
diff --git a/mlens/parallel/wrapper.py b/mlens/parallel/wrapper.py
index 4e67cc9b..15e3b7ac 100644
--- a/mlens/parallel/wrapper.py
+++ b/mlens/parallel/wrapper.py
@@ -165,9 +165,7 @@ def get_backend(instance):
     if _backend:
         instance = _backend
 
-    runnable = all([issubclass(instance.__class__, BaseParallel),
-                    issubclass(instance.__class__, OutputMixin)])
-    if runnable:
+    if issubclass(instance.__class__, BaseParallel):
         return instance
 
     raise ParallelProcessingError(
@@ -179,6 +177,8 @@ def set_flags(backend, flags):
     resets = list()
     if 'layer' in backend.__class__.__name__.lower():
         updates = [backend] + backend.learners
+    elif 'group' in backend.__class__.__name__.lower():
+        updates = backend.learners
     elif not isinstance(backend, list):
         updates = [backend]
     else:
@@ -237,16 +237,15 @@ def run(caller, job, X, y=None, map=True, **kwargs):
 
         out = mgr.map(caller, job, X, y, **kwargs)
 
-    Run allows dynamic parameter changing and resets, for instance running
-    a learner with ``proba=True`` on that previously had ``proba=False``.
+    :func:`run` handles temporary parameter changes, for instance running
+    a learner with ``proba=True`` that has ``proba=False`` as default.
     Similarly, instances destined to not produce output can be forced to
     yield predictions by passing ``return_preds=True`` as a keyword argument.
 
-    .. note:: Currently, :fun:`run` does not allow a :class:`Learner` to have a
-    preprocessing dependency. To achieve the same output, do::
-
-        caller([transformer, learner], map=False)
+    .. note:: To run a learner with a ``preprocessing`` dependency, the
+        instances need to be wrapped in a :class:`Group` ::
 
+            run(Group(learner, transformer), 'predict', X, y)
 
     Parameters
     ----------
@@ -282,9 +281,9 @@ def run(caller, job, X, y=None, map=True, **kwargs):
 
     try:
         verbose = max(getattr(caller, 'verbose', 0) - 4, 0)
-        backend = getattr(caller, 'backend', config.get_backend())
-        n_jobs = getattr(backend, 'n_jobs', -1)
-        with ParallelProcessing(backend, n_jobs, verbose) as mgr:
+        _backend = getattr(caller, 'backend', config.get_backend())
+        n_jobs = getattr(caller, 'n_jobs', -1)
+        with ParallelProcessing(_backend, n_jobs, verbose) as mgr:
             if map:
                 out = mgr.map(caller, job, X, y, **kwargs)
             else: