diff --git a/build/build_sepolicy.py b/build/build_sepolicy.py
new file mode 100644
index 0000000..ff2ff07
--- /dev/null
+++ b/build/build_sepolicy.py
@@ -0,0 +1,140 @@
+# Copyright 2018 - The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Command-line tool to build SEPolicy files."""
+
+import argparse
+import os
+import subprocess
+import sys
+
+import file_utils
+
+
+# All supported commands in this module.
+# For each command, need to add two functions. Take 'build_cil' for example:
+#   - setup_build_cil()
+#     - Sets up command parsers and sets default function to do_build_cil().
+#   - do_build_cil()
+_SUPPORTED_COMMANDS = ('build_cil',)
+
+
+def run_host_command(args, **kwargs):
+    """Runs a host command and prints output."""
+    if kwargs.get('shell'):
+        command_log = args
+    else:
+        command_log = ' '.join(args)  # For args as a sequence.
+
+    try:
+        subprocess.check_call(args, **kwargs)
+    except subprocess.CalledProcessError as err:
+        sys.stderr.write(
+            'build_sepolicy - failed to run command: {!r} (ret:{})\n'.format(
+                command_log, err.returncode))
+        sys.exit(err.returncode)
+
+
+def do_build_cil(args):
+    """Builds a sepolicy CIL (Common Intermediate Language) file.
+
+    This functions invokes some host utils (e.g., secilc, checkpolicy,
+    version_sepolicy) to generate a .cil file.
+
+    Args:
+        args: the parsed command arguments.
+    """
+    # Determines the raw CIL file name.
+    input_file_name = os.path.splitext(args.input_policy_conf)[0]
+    raw_cil_file = input_file_name + '_raw.cil'
+    # Builds the raw CIL.
+    file_utils.make_parent_dirs(raw_cil_file)
+    checkpolicy_cmd = [args.checkpolicy_env]
+    checkpolicy_cmd += [os.path.join(args.android_host_path, 'checkpolicy'),
+                        '-C', '-M', '-c', args.policy_vers,
+                        '-o', raw_cil_file, args.input_policy_conf]
+    # Using shell=True to setup args.checkpolicy_env variables.
+    run_host_command(' '.join(checkpolicy_cmd), shell=True)
+    file_utils.filter_out([args.reqd_mask], raw_cil_file)
+
+    # Builds the output CIL by versioning the above raw CIL.
+    output_file = args.output_cil
+    if output_file is None:
+        output_file = input_file_name + '.cil'
+    file_utils.make_parent_dirs(output_file)
+
+    run_host_command([os.path.join(args.android_host_path, 'version_policy'),
+                      '-b', args.base_policy, '-t', raw_cil_file,
+                      '-n', args.treble_sepolicy_vers, '-o', output_file])
+    if args.filter_out_files:
+        file_utils.filter_out(args.filter_out_files, output_file)
+
+    # Tests that the output file can be merged with the given CILs.
+    if args.dependent_cils:
+        merge_cmd = [os.path.join(args.android_host_path, 'secilc'),
+                     '-m', '-M', 'true', '-G', '-N', '-c', args.policy_vers]
+        merge_cmd += args.dependent_cils      # the give CILs to merge
+        merge_cmd += [output_file, '-o', '/dev/null', '-f', '/dev/null']
+        run_host_command(merge_cmd)
+
+
+def setup_build_cil(subparsers):
+    """Sets up command args for 'build_cil' command."""
+
+    # Required arguments.
+    parser = subparsers.add_parser('build_cil', help='build CIL files')
+    parser.add_argument('-i', '--input_policy_conf', required=True,
+                        help='source policy.conf')
+    parser.add_argument('-m', '--reqd_mask', required=True,
+                        help='the bare minimum policy.conf to use checkpolicy')
+    parser.add_argument('-b', '--base_policy', required=True,
+                        help='base policy for versioning')
+    parser.add_argument('-t', '--treble_sepolicy_vers', required=True,
+                        help='the version number to use for Treble-OTA')
+    parser.add_argument('-p', '--policy_vers', required=True,
+                        help='SELinux policy version')
+
+    # Optional arguments.
+    parser.add_argument('-c', '--checkpolicy_env',
+                        help='environment variables passed to checkpolicy')
+    parser.add_argument('-f', '--filter_out_files', nargs='+',
+                        help='the pattern files to filter out the output cil')
+    parser.add_argument('-d', '--dependent_cils', nargs='+',
+                        help=('check the output file can be merged with '
+                              'the dependent cil files'))
+    parser.add_argument('-o', '--output_cil', help='the output cil file')
+
+    # The function that performs the actual works.
+    parser.set_defaults(func=do_build_cil)
+
+
+def run(argv):
+    """Sets up command parser and execuates sub-command."""
+    parser = argparse.ArgumentParser()
+
+    # Adds top-level arguments.
+    parser.add_argument('-a', '--android_host_path', default='',
+                        help='a path to host out executables')
+
+    # Adds subparsers for each COMMAND.
+    subparsers = parser.add_subparsers(title='COMMAND')
+    for command in _SUPPORTED_COMMANDS:
+        globals()['setup_' + command](subparsers)
+
+    args = parser.parse_args(argv[1:])
+    args.func(args)
+
+
+if __name__ == '__main__':
+    run(sys.argv)