diff --git a/repo2docker/__main__.py b/repo2docker/__main__.py
index 58caaf64d..0e427221f 100644
--- a/repo2docker/__main__.py
+++ b/repo2docker/__main__.py
@@ -196,7 +196,6 @@ def get_argparser():
         '--cache-from',
         action='append',
         default=[],
-        #help=self.traits()['cache_from'].help
     )
 
     return argparser
@@ -208,7 +207,6 @@ def make_r2d(argv=None):
     if argv is None:
         argv = sys.argv[1:]
 
-
     # version must be checked before parse, as repo/cmd are required and
     # will spit out an error if allowed to be parsed first.
     if '--version' in argv:
@@ -244,9 +242,11 @@ def make_r2d(argv=None):
                             extra=dict(phase='failed'))
             sys.exit(1)
 
-
     if args.image_name:
         r2d.output_image_spec = args.image_name
+    else:
+        # we will pick a name after fetching the repository
+        r2d.output_image_spec = ""
 
     r2d.json_logs = args.json_logs
 
@@ -343,5 +343,6 @@ def main():
             r2d.log.exception(e)
         sys.exit(1)
 
+
 if __name__ == '__main__':
     main()
diff --git a/repo2docker/app.py b/repo2docker/app.py
index 12f9ace08..9e664ccda 100644
--- a/repo2docker/app.py
+++ b/repo2docker/app.py
@@ -13,7 +13,6 @@
 import logging
 import os
 import pwd
-import subprocess
 import shutil
 import tempfile
 import time
@@ -31,8 +30,7 @@
 from . import __version__
 from .buildpacks import (
     PythonBuildPack, DockerBuildPack, LegacyBinderDockerBuildPack,
-    CondaBuildPack, JuliaBuildPack, BaseImage,
-    RBuildPack, NixBuildPack
+    CondaBuildPack, JuliaBuildPack, RBuildPack, NixBuildPack
 )
 from . import contentproviders
 from .utils import (
@@ -364,7 +362,21 @@ def fetch(self, url, ref, checkout_path):
                 spec, checkout_path, yield_output=self.json_logs):
             self.log.info(log_line, extra=dict(phase='fetching'))
 
-
+        if not self.output_image_spec:
+            self.output_image_spec = (
+                'r2d' + escapism.escape(self.repo, escape_char='-').lower()
+                )
+            # if we are building from a subdirectory include that in the
+            # image name so we can tell builds from different sub-directories
+            # apart.
+            if self.subdir:
+                self.output_image_spec += (
+                    escapism.escape(self.subdir, escape_char='-').lower()
+                )
+            if picked_content_provider.content_id is not None:
+                self.output_image_spec += picked_content_provider.content_id
+            else:
+                self.output_image_spec += str(int(time.time()))
 
     def json_excepthook(self, etype, evalue, traceback):
         """Called on an uncaught exception when using json logging
@@ -399,15 +411,6 @@ def initialize(self):
                 fmt='%(message)s'
             )
 
-        if self.output_image_spec == "":
-            # Attempt to set a sane default!
-            # HACK: Provide something more descriptive?
-            self.output_image_spec = (
-                'r2d' +
-                escapism.escape(self.repo, escape_char='-').lower() +
-                str(int(time.time()))
-            )
-
         if self.dry_run and (self.run or self.push):
             raise ValueError("Cannot push or run image if we are not building it")
 
@@ -546,6 +549,20 @@ def _get_free_port(self):
         s.close()
         return port
 
+    def find_image(self):
+        # if this is a dry run it is Ok for dockerd to be unreachable so we
+        # always return False for dry runs.
+        if self.dry_run:
+            return False
+        # check if we already have an image for this content
+        client = docker.APIClient(version='auto', **kwargs_from_env())
+        for image in client.images():
+            if image['RepoTags'] is not None:
+                for tag in image['RepoTags']:
+                    if tag == self.output_image_spec + ":latest":
+                        return True
+        return False
+
     def build(self):
         """
         Build docker image
@@ -553,8 +570,8 @@ def build(self):
         # Check if r2d can connect to docker daemon
         if not self.dry_run:
             try:
-                api_client = docker.APIClient(version='auto',
-                                              **kwargs_from_env())
+                docker_client = docker.APIClient(version='auto',
+                                                 **kwargs_from_env())
             except DockerException as e:
                 self.log.exception(e)
                 raise
@@ -574,6 +591,14 @@ def build(self):
         try:
             self.fetch(self.repo, self.ref, checkout_path)
 
+            if self.find_image():
+                self.log.info("Reusing existing image ({}), not "
+                              "building.".format(self.output_image_spec))
+                # no need to build, so skip to the end by `return`ing here
+                # this will still execute the finally clause and let's us
+                # avoid having to indent the build code by an extra level
+                return
+
             if self.subdir:
                 checkout_path = os.path.join(checkout_path, self.subdir)
                 if not os.path.isdir(checkout_path):
@@ -610,8 +635,11 @@ def build(self):
                     self.log.info('Using %s builder\n', bp.__class__.__name__,
                                   extra=dict(phase='building'))
 
-                    for l in picked_buildpack.build(api_client, self.output_image_spec,
-                        self.build_memory_limit, build_args, self.cache_from):
+                    for l in picked_buildpack.build(docker_client,
+                                                    self.output_image_spec,
+                                                    self.build_memory_limit,
+                                                    build_args,
+                                                    self.cache_from):
                         if 'stream' in l:
                             self.log.info(l['stream'],
                                           extra=dict(phase='building'))
@@ -624,8 +652,9 @@ def build(self):
                         else:
                             self.log.info(json.dumps(l),
                                           extra=dict(phase='building'))
+
         finally:
-            # Cheanup checkout if necessary
+            # Cleanup checkout if necessary
             if self.cleanup_checkout:
                 shutil.rmtree(checkout_path, ignore_errors=True)
 
diff --git a/repo2docker/contentproviders/base.py b/repo2docker/contentproviders/base.py
index 9b9863258..7cdf0d5b0 100644
--- a/repo2docker/contentproviders/base.py
+++ b/repo2docker/contentproviders/base.py
@@ -17,6 +17,20 @@ class ContentProvider:
     def __init__(self):
         self.log = logging.getLogger("repo2docker")
 
+    @property
+    def content_id(self):
+        """A unique ID to represent the version of the content.
+        This ID is used to name the built images. If the ID is the same between
+        two runs of repo2docker we will reuse an existing image (if it exists).
+        By providing an ID that summarizes the content we can reuse existing
+        images and speed up build times. A good ID is the revision of a Git
+        repository or a hash computed from all the content.
+        The type content ID can be any string.
+        To disable this behaviour set this property to `None` in which case
+        a fresh image will always be built.
+        """
+        return None
+
     def detect(self, repo, ref=None, extra_args=None):
         """Determine compatibility between source and this provider.
 
diff --git a/repo2docker/contentproviders/git.py b/repo2docker/contentproviders/git.py
index 1060a670c..fdade7d0e 100644
--- a/repo2docker/contentproviders/git.py
+++ b/repo2docker/contentproviders/git.py
@@ -44,3 +44,14 @@ def fetch(self, spec, output_dir, yield_output=False):
                                     cwd=output_dir,
                                     capture=yield_output):
                 yield line
+
+        cmd = ['git', 'rev-parse', 'HEAD']
+        sha1 = subprocess.Popen(cmd, stdout=subprocess.PIPE, cwd=output_dir)
+        self._sha1 = sha1.stdout.read().decode().strip()
+
+    @property
+    def content_id(self):
+        """A unique ID to represent the version of the content.
+        Uses the first seven characters of the git commit ID of the repository.
+        """
+        return self._sha1[:7]
diff --git a/tests/conftest.py b/tests/conftest.py
index 36112b5bd..8d0fa5e5e 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -12,8 +12,11 @@
 import pipes
 import shlex
 import requests
+import subprocess
 import time
 
+from tempfile import TemporaryDirectory
+
 import pytest
 import yaml
 
@@ -77,6 +80,35 @@ def run_test(args):
     return run_test
 
 
+@pytest.fixture()
+def git_repo():
+    """
+    Make a dummy git repo in which user can perform git operations
+
+    Should be used as a contextmanager, it will delete directory when done
+    """
+    with TemporaryDirectory() as gitdir:
+        subprocess.check_call(['git', 'init'], cwd=gitdir)
+        yield gitdir
+
+
+@pytest.fixture()
+def repo_with_content(git_repo):
+    """Create a git repository with content"""
+    with open(os.path.join(git_repo, 'test'), 'w') as f:
+        f.write("Hello")
+
+    subprocess.check_call(['git', 'add', 'test'], cwd=git_repo)
+    subprocess.check_call(['git', 'commit', '-m', 'Test commit'],
+                          cwd=git_repo)
+    # get the commit's SHA1
+    sha1 = subprocess.Popen(['git', 'rev-parse', 'HEAD'],
+                            stdout=subprocess.PIPE, cwd=git_repo)
+    sha1 = sha1.stdout.read().decode().strip()
+
+    yield git_repo, sha1
+
+
 class Repo2DockerTest(pytest.Function):
     """A pytest.Item for running repo2docker"""
     def __init__(self, name, parent, args):
diff --git a/tests/unit/contentproviders/test_git.py b/tests/unit/contentproviders/test_git.py
index 023e2e490..e377f5621 100644
--- a/tests/unit/contentproviders/test_git.py
+++ b/tests/unit/contentproviders/test_git.py
@@ -1,50 +1,34 @@
-from contextlib import contextmanager
 import os
-import subprocess
 import pytest
 from tempfile import TemporaryDirectory
 from repo2docker.contentproviders import Git
 
 
-@contextmanager
-def git_repo():
-    """
-    Makes a dummy git repo in which user can perform git operations
-
-    Should be used as a contextmanager, it will delete directory when done
-    """
-
-    with TemporaryDirectory() as gitdir:
-        subprocess.check_call(['git', 'init'], cwd=gitdir)
-        yield gitdir
-
-
-def test_clone():
+def test_clone(repo_with_content):
     """Test simple git clone to a target dir"""
-    with git_repo() as upstream:
-        with open(os.path.join(upstream, 'test'), 'w') as f:
-            f.write("Hello")
+    upstream, sha1 = repo_with_content
 
-        subprocess.check_call(['git', 'add', 'test'], cwd=upstream)
-        subprocess.check_call(['git', 'commit', '-m', 'Test commit'],
-                              cwd=upstream)
+    with TemporaryDirectory() as clone_dir:
+        spec = {'repo': upstream}
+        git_content = Git()
+        for _ in git_content.fetch(spec, clone_dir):
+            pass
+        assert os.path.exists(os.path.join(clone_dir, 'test'))
+
+        assert git_content.content_id == sha1[:7]
 
-        with TemporaryDirectory() as clone_dir:
-            spec = {'repo': upstream}
-            for _ in Git().fetch(spec, clone_dir):
-                pass
-            assert os.path.exists(os.path.join(clone_dir, 'test'))
 
-def test_bad_ref():
+def test_bad_ref(repo_with_content):
     """
     Test trying to checkout a ref that doesn't exist
     """
-    with git_repo() as upstream:
-        with TemporaryDirectory() as clone_dir:
-            spec = {'repo': upstream, 'ref': 'does-not-exist'}
-            with pytest.raises(ValueError):
-                for _ in Git().fetch(spec, clone_dir):
-                    pass
+    upstream, sha1 = repo_with_content
+    with TemporaryDirectory() as clone_dir:
+        spec = {'repo': upstream, 'ref': 'does-not-exist'}
+        with pytest.raises(ValueError):
+            for _ in Git().fetch(spec, clone_dir):
+                pass
+
 
 def test_always_accept():
     # The git content provider should always accept a spec
diff --git a/tests/unit/contentproviders/test_local.py b/tests/unit/contentproviders/test_local.py
index 8212adef9..62b5eddb8 100644
--- a/tests/unit/contentproviders/test_local.py
+++ b/tests/unit/contentproviders/test_local.py
@@ -24,6 +24,13 @@ def test_not_detect_local_file():
         assert spec is None, spec
 
 
+def test_content_id_is_None():
+    # content_id property should always be None for local content provider
+    # as we rely on the caching done by docker
+    local = Local()
+    assert local.content_id is None
+
+
 def test_content_available():
     # create a directory with files, check they are available in the output
     # directory
@@ -31,7 +38,11 @@ def test_content_available():
         with open(os.path.join(d, 'test'), 'w') as f:
             f.write("Hello")
 
+        local = Local()
         spec = {'path': d}
-        for _ in Local().fetch(spec, d):
+        for _ in local.fetch(spec, d):
             pass
         assert os.path.exists(os.path.join(d, 'test'))
+        # content_id property should always be None for local content provider
+        # as we rely on the caching done by docker
+        assert local.content_id is None
diff --git a/tests/unit/test_app.py b/tests/unit/test_app.py
new file mode 100644
index 000000000..e828ed9ce
--- /dev/null
+++ b/tests/unit/test_app.py
@@ -0,0 +1,74 @@
+from tempfile import TemporaryDirectory
+from unittest.mock import patch
+
+import escapism
+
+from repo2docker.app import Repo2Docker
+from repo2docker.__main__ import make_r2d
+
+
+def test_find_image():
+    images = [{'RepoTags': ['some-org/some-repo:latest']}]
+
+    with patch('repo2docker.app.docker.APIClient') as FakeDockerClient:
+        instance = FakeDockerClient.return_value
+        instance.images.return_value = images
+
+        r2d = Repo2Docker()
+        r2d.output_image_spec = 'some-org/some-repo'
+        assert r2d.find_image()
+
+        instance.images.assert_called_with()
+
+
+def test_dont_find_image():
+    images = [{'RepoTags': ['some-org/some-image-name:latest']}]
+
+    with patch('repo2docker.app.docker.APIClient') as FakeDockerClient:
+        instance = FakeDockerClient.return_value
+        instance.images.return_value = images
+
+        r2d = Repo2Docker()
+        r2d.output_image_spec = 'some-org/some-other-image-name'
+        assert not r2d.find_image()
+
+        instance.images.assert_called_with()
+
+
+def test_image_name_remains_unchanged():
+    # if we specify an image name, it should remain unmodified
+    with TemporaryDirectory() as src:
+        app = Repo2Docker()
+        argv = ['--image-name', 'a-special-name', '--no-build', src]
+        app = make_r2d(argv)
+
+        app.start()
+
+        assert app.output_image_spec == 'a-special-name'
+
+
+def test_image_name_contains_sha1(repo_with_content):
+    upstream, sha1 = repo_with_content
+    app = Repo2Docker()
+    # force selection of the git content provider by prefixing path with
+    # file://. This is important as the Local content provider does not
+    # store the SHA1 in the repo spec
+    argv = ['--no-build', 'file://' + upstream]
+    app = make_r2d(argv)
+
+    app.start()
+
+    assert app.output_image_spec.endswith(sha1[:7])
+
+
+def test_local_dir_image_name(repo_with_content):
+    upstream, sha1 = repo_with_content
+    app = Repo2Docker()
+    argv = ['--no-build', upstream]
+    app = make_r2d(argv)
+
+    app.start()
+
+    assert app.output_image_spec.startswith(
+        'r2d' + escapism.escape(upstream, escape_char='-').lower()
+    )
diff --git a/tests/unit/test_memlimit.py b/tests/unit/test_memlimit.py
index b8c53c6c3..f636da7ce 100644
--- a/tests/unit/test_memlimit.py
+++ b/tests/unit/test_memlimit.py
@@ -66,7 +66,6 @@ def test_memlimit_nondockerfile(tmpdir, test, mem_limit, mem_allocate_mb, expect
     assert success == expected
 
 
-
 def test_memlimit_same_postbuild():
     """
     Validate that the postBuild files for dockerfile & nondockerfile are same
diff --git a/tests/unit/test_subdir.py b/tests/unit/test_subdir.py
index 89f0884b3..b33e91278 100644
--- a/tests/unit/test_subdir.py
+++ b/tests/unit/test_subdir.py
@@ -2,7 +2,8 @@
 Test if the subdirectory is correctly navigated to
 """
 import os
-import logging
+
+import escapism
 
 import pytest
 from repo2docker.app import Repo2Docker
@@ -23,10 +24,20 @@ def test_subdir(run_repo2docker):
     assert cwd == os.getcwd(), "We should be back in %s" % cwd
 
 
+def test_subdir_in_image_name():
+    app = Repo2Docker(
+        repo=TEST_REPO,
+        subdir='a directory',
+    )
+    app.initialize()
+    app.build()
+
+    escaped_dirname = escapism.escape('a directory', escape_char='-').lower()
+    assert escaped_dirname in app.output_image_spec
+
+
 def test_subdir_invalid(caplog):
     # test an error is raised when requesting a non existent subdir
-    #caplog.set_level(logging.INFO, logger='Repo2Docker')
-
     app = Repo2Docker(
         repo=TEST_REPO,
         subdir='invalid-sub-dir',