diff --git a/pre_commit/commands/install.py b/pre_commit/commands/install.py
index d208ff7f..606cfd25 100644
--- a/pre_commit/commands/install.py
+++ b/pre_commit/commands/install.py
@@ -1,24 +1,44 @@
 from __future__ import print_function
 from __future__ import unicode_literals
 
+import io
 import os
+import os.path
 import pkg_resources
 import stat
 
+# This is used to identify the hook file we install
+IDENTIFYING_HASH = 'd8ee923c46731b42cd95cc869add4062'
+
+
+def is_our_pre_commit(filename):
+    return IDENTIFYING_HASH in io.open(filename).read()
+
+
+def make_executable(filename):
+    original_mode = os.stat(filename).st_mode
+    os.chmod(
+        filename,
+        original_mode | stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH,
+    )
+
 
 def install(runner):
     """Install the pre-commit hooks."""
     pre_commit_file = pkg_resources.resource_filename(
         'pre_commit', 'resources/pre-commit-hook',
     )
+
+    # If we have an existing hook, move it to pre-commit.legacy
+    if (
+        os.path.exists(runner.pre_commit_path) and
+        not is_our_pre_commit(runner.pre_commit_path)
+    ):
+        os.rename(runner.pre_commit_path, runner.pre_commit_path + '.legacy')
+
     with open(runner.pre_commit_path, 'w') as pre_commit_file_obj:
         pre_commit_file_obj.write(open(pre_commit_file).read())
-
-    original_mode = os.stat(runner.pre_commit_path).st_mode
-    os.chmod(
-        runner.pre_commit_path,
-        original_mode | stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH,
-    )
+    make_executable(runner.pre_commit_path)
 
     print('pre-commit installed at {0}'.format(runner.pre_commit_path))
     return 0
diff --git a/pre_commit/resources/pre-commit-hook b/pre_commit/resources/pre-commit-hook
index 1a93b653..60c620cf 100755
--- a/pre_commit/resources/pre-commit-hook
+++ b/pre_commit/resources/pre-commit-hook
@@ -1,4 +1,10 @@
 #!/usr/bin/env bash
+# This is a randomish md5 to identify this script
+# d8ee923c46731b42cd95cc869add4062
+
+HERE=$(dirname $(readlink -f "$0"))
+
+retv=0
 
 which pre-commit > /dev/null
 if [ $? -ne 0 ]; then
@@ -6,4 +12,20 @@ if [ $? -ne 0 ]; then
     exit 1
 fi
 
+
+# Run the legacy pre-commit if it exists
+if [ -x "$HERE"/pre-commit.legacy ]; then
+    "$HERE"/pre-commit.legacy
+    if [ $? -ne 0 ]; then
+        retv=1
+    fi
+fi
+
+
+# Run pre-commit
 pre-commit
+if [ $? -ne 0 ]; then
+    retv=1
+fi
+
+exit $retv
diff --git a/tests/commands/install_test.py b/tests/commands/install_test.py
index d87356e6..00bca826 100644
--- a/tests/commands/install_test.py
+++ b/tests/commands/install_test.py
@@ -11,14 +11,44 @@ import stat
 from plumbum import local
 
 from pre_commit.commands.install import install
+from pre_commit.commands.install import is_our_pre_commit
+from pre_commit.commands.install import make_executable
 from pre_commit.runner import Runner
 from testing.fixtures import git_dir
 from testing.fixtures import make_consuming_repo
 
 
-def _get_commit_output(tmpdir_factory):
-    local['touch']('foo')
-    local['git']('add', 'foo')
+def test_is_not_our_pre_commit():
+    assert is_our_pre_commit('setup.py') is False
+
+
+def test_is_our_pre_commit():
+    assert is_our_pre_commit(
+        pkg_resources.resource_filename(
+            'pre_commit', 'resources/pre-commit-hook',
+        )
+    ) is True
+
+
+def test_install_pre_commit(tmpdir_factory):
+    path = git_dir(tmpdir_factory)
+    runner = Runner(path)
+    ret = install(runner)
+    assert ret == 0
+    assert os.path.exists(runner.pre_commit_path)
+    pre_commit_contents = io.open(runner.pre_commit_path).read()
+    pre_commit_script = pkg_resources.resource_filename(
+        'pre_commit', 'resources/pre-commit-hook',
+    )
+    expected_contents = io.open(pre_commit_script).read()
+    assert pre_commit_contents == expected_contents
+    stat_result = os.stat(runner.pre_commit_path)
+    assert stat_result.st_mode & (stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH)
+
+
+def _get_commit_output(tmpdir_factory, touch_file='foo'):
+    local['touch'](touch_file)
+    local['git']('add', touch_file)
     # Don't want to write to home directory
     env = dict(os.environ, **{'PRE_COMMIT_HOME': tmpdir_factory.get()})
     return local['git'].run(
@@ -41,34 +71,6 @@ NORMAL_PRE_COMMIT_RUN = re.compile(
 )
 
 
-FAILING_PRE_COMMIT_RUN = re.compile(
-    r'^\[INFO\] Installing environment for .+\.\n'
-    r'\[INFO\] Once installed this environment will be reused\.\n'
-    r'\[INFO\] This may take a few minutes\.\.\.\n'
-    r'Failing hook\.+Failed\n'
-    r'\n'
-    r'Fail\n'
-    r'foo\n'
-    r'\n$'
-)
-
-
-def test_install_pre_commit(tmpdir_factory):
-    path = git_dir(tmpdir_factory)
-    runner = Runner(path)
-    ret = install(runner)
-    assert ret == 0
-    assert os.path.exists(runner.pre_commit_path)
-    pre_commit_contents = io.open(runner.pre_commit_path).read()
-    pre_commit_sh = pkg_resources.resource_filename(
-        'pre_commit', 'resources/pre-commit-hook',
-    )
-    expected_contents = io.open(pre_commit_sh).read()
-    assert pre_commit_contents == expected_contents
-    stat_result = os.stat(runner.pre_commit_path)
-    assert stat_result.st_mode & (stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH)
-
-
 def test_install_pre_commit_and_run(tmpdir_factory):
     path = make_consuming_repo(tmpdir_factory, 'script_hooks_repo')
     with local.cwd(path):
@@ -108,6 +110,18 @@ def test_environment_not_sourced(tmpdir_factory):
         )
 
 
+FAILING_PRE_COMMIT_RUN = re.compile(
+    r'^\[INFO\] Installing environment for .+\.\n'
+    r'\[INFO\] Once installed this environment will be reused\.\n'
+    r'\[INFO\] This may take a few minutes\.\.\.\n'
+    r'Failing hook\.+Failed\n'
+    r'\n'
+    r'Fail\n'
+    r'foo\n'
+    r'\n$'
+)
+
+
 def test_failing_hooks_returns_nonzero(tmpdir_factory):
     path = make_consuming_repo(tmpdir_factory, 'failing_hook_repo')
     with local.cwd(path):
@@ -116,3 +130,63 @@ def test_failing_hooks_returns_nonzero(tmpdir_factory):
         ret, output = _get_commit_output(tmpdir_factory)
         assert ret == 1
         assert FAILING_PRE_COMMIT_RUN.match(output)
+
+
+EXISTING_COMMIT_RUN = re.compile(
+    r'^legacy hook\n'
+    r'\[master [a-f0-9]{7}\] Commit!\n'
+    r' 0 files changed\n'
+    r' create mode 100644 baz\n$'
+)
+
+
+def test_install_existing_hooks_no_overwrite(tmpdir_factory):
+    path = make_consuming_repo(tmpdir_factory, 'script_hooks_repo')
+    with local.cwd(path):
+        runner = Runner(path)
+
+        # Write out an "old" hook
+        with io.open(runner.pre_commit_path, 'w') as hook_file:
+            hook_file.write('#!/usr/bin/env bash\necho "legacy hook"\n')
+        make_executable(runner.pre_commit_path)
+
+        # Make sure we installed the "old" hook correctly
+        ret, output = _get_commit_output(tmpdir_factory, touch_file='baz')
+        assert ret == 0
+        assert EXISTING_COMMIT_RUN.match(output)
+
+        # Now install pre-commit (no-overwrite)
+        assert install(Runner(path)) == 0
+
+        # We should run both the legacy and pre-commit hooks
+        ret, output = _get_commit_output(tmpdir_factory)
+        assert ret == 0
+        assert output.startswith('legacy hook\n')
+        assert NORMAL_PRE_COMMIT_RUN.match(output[len('legacy hook\n'):])
+
+
+FAIL_OLD_HOOK = re.compile(
+    r'fail!\n'
+    r'\[INFO\] Installing environment for .+\.\n'
+    r'\[INFO\] Once installed this environment will be reused\.\n'
+    r'\[INFO\] This may take a few minutes\.\.\.\n'
+    r'Bash hook\.+Passed\n'
+)
+
+
+def test_failing_existing_hook_returns_1(tmpdir_factory):
+    path = make_consuming_repo(tmpdir_factory, 'script_hooks_repo')
+    with local.cwd(path):
+        runner = Runner(path)
+
+        # Write out a failing "old" hook
+        with io.open(runner.pre_commit_path, 'w') as hook_file:
+            hook_file.write('#!/usr/bin/env bash\necho "fail!"\nexit 1\n')
+        make_executable(runner.pre_commit_path)
+
+        assert install(Runner(path)) == 0
+
+        # We should get a failure from the legacy hook
+        ret, output = _get_commit_output(tmpdir_factory)
+        assert ret == 1
+        assert FAIL_OLD_HOOK.match(output)