diff --git a/scripts/diff_build_graphs.sh b/scripts/diff_build_graphs.sh
new file mode 100755
index 0000000..e7d8749
--- /dev/null
+++ b/scripts/diff_build_graphs.sh
@@ -0,0 +1,170 @@
+#!/bin/bash -eu
+#
+# Copyright 2017 Google Inc. All rights reserved.
+#
+# 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.
+
+set -e
+
+# This file makes it easy to confirm that a set of changes in source code don't result in any
+# changes to the generated ninja files. This is to reduce the effort required to be confident
+# in the correctness of refactorings
+
+function die() {
+  echo "$@" >&2
+  exit 1
+}
+
+function usage() {
+  violation="$1"
+  die "$violation
+
+  Usage: diff_build_graphs.sh [--products=product1,product2...] <OLD_VERSIONS> <NEW_VERSIONS>
+
+  This file builds and parses the build files (Android.mk, Android.bp, etc) for each requested
+  product and for both sets of versions, and checks whether the ninja files (which implement
+  the build graph) changed between the two versions.
+
+  Example: diff_build_graphs.sh 'build/soong:work^ build/blueprint:work^' 'build/soong:work build/blueprint:work'
+
+  Options:
+    --products=PRODUCTS  comma-separated list of products to check"
+}
+
+PRODUCTS_ARG=""
+OLD_VERSIONS=""
+NEW_VERSIONS=""
+function parse_args() {
+  # parse optional arguments
+  while true; do
+    arg="${1-}"
+    case "$arg" in
+      --products=*) PRODUCTS_ARG="$arg";;
+      *) break;;
+    esac
+    shift
+  done
+  # parse required arguments
+  if [ "$#" != "2" ]; then
+    usage ""
+  fi
+  #argument validation
+  OLD_VERSIONS="$1"
+  NEW_VERSIONS="$2"
+
+}
+parse_args "$@"
+
+
+# find some file paths
+cd "$(dirname $0)"
+SCRIPT_DIR="$PWD"
+cd ../../..
+CHECKOUT_ROOT="$PWD"
+OUT_DIR="${OUT_DIR-}"
+if [ -z "$OUT_DIR" ]; then
+  OUT_DIR=out
+fi
+WORK_DIR="$OUT_DIR/diff"
+OUT_DIR_OLD="$WORK_DIR/out_old"
+OUT_DIR_NEW="$WORK_DIR/out_new"
+OUT_DIR_TEMP="$WORK_DIR/out_temp"
+
+
+function checkout() {
+  versionSpecs="$1"
+  for versionSpec in $versionSpecs; do
+    project="$(echo $versionSpec | sed 's|\([^:]*\):\([^:]*\)|\1|')"
+    ref="$(echo     $versionSpec | sed 's|\([^:]*\):\([^:]*\)|\2|')"
+    echo "checking out ref $ref in project $project"
+    git -C "$project" checkout "$ref"
+  done
+}
+
+function run_build() {
+  echo
+  echo "Starting build"
+  # rebuild multiproduct_kati, in case it was missing before,
+  # or in case it is affected by some of the changes we're testing
+  make blueprint_tools
+  # find multiproduct_kati and have it build the ninja files for each product
+  builder="$(echo $OUT_DIR/soong/host/*/bin/multiproduct_kati)"
+  BUILD_NUMBER=sample "$builder" $PRODUCTS_ARG --keep --out "$OUT_DIR_TEMP" || true
+  echo
+}
+
+function diffProduct() {
+  product="$1"
+
+  zip1="$OUT_DIR_OLD/${product}.zip"
+  unzipped1="$OUT_DIR_OLD/$product"
+
+  zip2="$OUT_DIR_NEW/${product}.zip"
+  unzipped2="$OUT_DIR_NEW/$product"
+
+  unzip -qq "$zip1" -d "$unzipped1"
+  unzip -qq "$zip2" -d "$unzipped2"
+
+  #do a diff of the ninja files
+  diffFile="$WORK_DIR/diff.txt"
+  diff -r "$unzipped1" "$unzipped2" -x build_date.txt -x build_number.txt -x '\.*' -x '*.log' -x build_fingerprint.txt -x build.ninja.d -x '*.zip' > $diffFile || true
+  if [[ -s "$diffFile" ]]; then
+    # outputs are different, so remove the unzipped versions but keep the zipped versions
+    echo "Some differences for product $product:"
+    cat "$diffFile" | head -n 10
+    echo "End of differences for product $product"
+    rm -rf "$unzipped1" "$unzipped2"
+  else
+    # outputs are the same, so remove all of the outputs
+    rm -rf "$zip1" "$unzipped1" "$zip2" "$unzipped2"
+  fi
+}
+
+function do_builds() {
+  #reset work dir
+  rm -rf "$WORK_DIR"
+  mkdir "$WORK_DIR"
+
+  #build new code
+  checkout "$NEW_VERSIONS"
+  run_build
+  mv "$OUT_DIR_TEMP" "$OUT_DIR_NEW"
+
+  #build old code
+  #TODO do we want to cache old results? Maybe by the time we care to cache old results this will
+  #be running on a remote server somewhere and be completely different
+  checkout "$OLD_VERSIONS"
+  run_build
+  mv "$OUT_DIR_TEMP" "$OUT_DIR_OLD"
+
+  #cleanup
+  echo created "$OUT_DIR_OLD" and "$OUT_DIR_NEW"
+}
+
+function main() {
+  do_builds
+  checkout "$NEW_VERSIONS"
+
+  #find all products
+  productsFile="$WORK_DIR/all_products.txt"
+  find $OUT_DIR_OLD $OUT_DIR_NEW -mindepth 1 -maxdepth 1 -name "*.zip" | sed "s|^$OUT_DIR_OLD/||" | sed "s|^$OUT_DIR_NEW/||" | sed "s|\.zip$||" | sort | uniq > "$productsFile"
+  echo Diffing products
+  for product in $(cat $productsFile); do
+    diffProduct "$product"
+  done
+  echo Done diffing products
+  echo "Any differing outputs can be seen at $OUT_DIR_OLD/*.zip and $OUT_DIR_NEW/*.zip"
+  echo "See $WORK_DIR/diff.txt for the full list of differences for the latest product checked"
+}
+
+main