diff --git a/.npmignore b/.npmignore
index e8add85..bf4749d 100755
--- a/.npmignore
+++ b/.npmignore
@@ -1,3 +1,4 @@
+fixtures
 src
 tests
 coverage
diff --git a/README.md b/README.md
index 8cdfbcf..4918e92 100644
--- a/README.md
+++ b/README.md
@@ -6,6 +6,81 @@
 
 `create-index` program creates (and maintains) ES6 `./index.js` file in target directories that imports and exports sibling files and directories.
 
+## Usage
+
+### Using CLI Program
+
+```sh
+npm install create-index
+
+create-index --help
+
+Options:
+  --update-index  Recursively iterates target directories looking for "index.js"
+                  files that start with "'create index';\n" (create-index index
+                  file). Updates found index files. Does not create new index
+                  files.                              [boolean] [default: false]
+
+Examples:
+  create-index ./src ./src/utilities        Creates or updates an existing
+                                            create-index index file in the
+                                            target (./src, ./src/utilities)
+                                            directories.
+  create-index --update-index ./src         Finds all create-index index files
+  ./tests                                   in the target directories and
+                                            descending directories. Updates
+                                            found index files.
+```
+
+### Using `create-index` Programmatically
+
+```js
+import {
+    writeIndex
+} from 'create-index';
+
+/**
+ * @type {Function}
+ * @param {Array<string>} directoryPaths
+ * @throws {Error} Directory "..." does not exist.
+ * @throws {Error} "..." is not a directory.
+ * @throws {Error} "..." unsafe index.
+ * @returns {boolean}
+ */
+writeIndex;
+```
+
+Note that the `writeIndex` function is synchronous.
+
+```js
+import {
+    findIndexFiles
+} from 'create-index';
+
+/**
+ * @type {Function}
+ * @param {string} directoryPath
+ * @returns {Array<string>} List of directory paths that have create-index index file.
+ */
+findIndexFiles;
+```
+
+### Gulp
+
+Since [Gulp](http://gulpjs.com/) can ran arbitrary JavaScript code, there is no need for a separate plugin. See [Using `create-index` Programmatically](#using-create-index-programmatically).
+
+```js
+import {
+    writeIndex
+} from 'create-index';
+
+gulp.task('create-index', () => {
+    writeIndex(['./target_directory']);
+});
+```
+
+Note that the `writeIndex` function is synchronous.
+
 ## Implementation
 
 `create-index` program will look into the target directory.
@@ -51,49 +126,3 @@ Directories that do not have `./index.js` in themselves will be excluded.
 When run again, `create-index` will update existing `./index.js` if it starts with `'create index';\n\n`.
 
 If `create-index` is executed against a directory that contains `./index.js`, which does not start with `'create index';\n\n`, an error will be thrown.
-
-## Usage
-
-### Using CLI Program
-
-```sh
-npm install create-index
-
-create-index ./directory1 ./directory2 ./directory3 ...
-```
-
-### Using `create-index` Programmatically
-
-```js
-import {
-    writeIndex
-} from 'create-index';
-
-/**
- * @type {Function}
- * @param {Array<string>} directoryPaths
- * @throws {Error} Directory "..." does not exist.
- * @throws {Error} "..." is not a directory.
- * @throws {Error} "..." unsafe index.
- * @returns {boolean}
- */
-writeIndex;
-```
-
-Note that the `writeIndex` function is synchronous.
-
-### Gulp
-
-Since [Gulp](http://gulpjs.com/) can ran arbitrary JavaScript code, there is no need for a separate plugin. See [Using `create-index` Programmatically](#using-create-index-programmatically).
-
-```js
-import {
-    writeIndex
-} from 'create-index';
-
-gulp.task('create-index', () => {
-    writeIndex(['./target_directory']);
-});
-```
-
-Note that the `writeIndex` function is synchronous.
diff --git a/tests/fixtures/read-directory/children-directories-and-files/foo/index.js b/fixtures/find-index-files/do-not-find-no-index/not-index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-and-files/foo/index.js
rename to fixtures/find-index-files/do-not-find-no-index/not-index.js
diff --git a/fixtures/find-index-files/do-not-find/index.js b/fixtures/find-index-files/do-not-find/index.js
new file mode 100644
index 0000000..e884c1f
--- /dev/null
+++ b/fixtures/find-index-files/do-not-find/index.js
@@ -0,0 +1 @@
+// 'create index';
diff --git a/tests/fixtures/read-directory/children-directories-unsafe-name/bar-bar/index.js b/fixtures/find-index-files/find-1/index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-unsafe-name/bar-bar/index.js
rename to fixtures/find-index-files/find-1/index.js
diff --git a/fixtures/find-index-files/nested/do-not-find/index.js b/fixtures/find-index-files/nested/do-not-find/index.js
new file mode 100644
index 0000000..e884c1f
--- /dev/null
+++ b/fixtures/find-index-files/nested/do-not-find/index.js
@@ -0,0 +1 @@
+// 'create index';
diff --git a/tests/fixtures/read-directory/children-directories-unsafe-name/foo-foo/index.js b/fixtures/find-index-files/nested/find-2/index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-unsafe-name/foo-foo/index.js
rename to fixtures/find-index-files/nested/find-2/index.js
diff --git a/tests/fixtures/read-directory/children-directories-unsafe-name/present/index.js b/fixtures/find-index-files/nested/find-3/index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-unsafe-name/present/index.js
rename to fixtures/find-index-files/nested/find-3/index.js
diff --git a/tests/fixtures/read-directory/children-directories-and-files/foo.js b/fixtures/read-directory/children-directories-and-files/foo.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-and-files/foo.js
rename to fixtures/read-directory/children-directories-and-files/foo.js
diff --git a/tests/fixtures/read-directory/children-directories/bar/index.js b/fixtures/read-directory/children-directories-and-files/foo/index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories/bar/index.js
rename to fixtures/read-directory/children-directories-and-files/foo/index.js
diff --git a/tests/fixtures/read-directory/children-directories-and-files/present.js b/fixtures/read-directory/children-directories-and-files/present.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-and-files/present.js
rename to fixtures/read-directory/children-directories-and-files/present.js
diff --git a/tests/fixtures/read-directory/children-directories/foo/index.js b/fixtures/read-directory/children-directories-unsafe-name/bar-bar/index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories/foo/index.js
rename to fixtures/read-directory/children-directories-unsafe-name/bar-bar/index.js
diff --git a/tests/fixtures/read-directory/children-index/bar/index.js b/fixtures/read-directory/children-directories-unsafe-name/foo-foo/index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-index/bar/index.js
rename to fixtures/read-directory/children-directories-unsafe-name/foo-foo/index.js
diff --git a/tests/fixtures/read-directory/children-index/foo/index.js b/fixtures/read-directory/children-directories-unsafe-name/present/index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-index/foo/index.js
rename to fixtures/read-directory/children-directories-unsafe-name/present/index.js
diff --git a/tests/fixtures/read-directory/children-directories-without-index/bar/bar.js b/fixtures/read-directory/children-directories-without-index/bar/bar.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-without-index/bar/bar.js
rename to fixtures/read-directory/children-directories-without-index/bar/bar.js
diff --git a/tests/fixtures/read-directory/children-directories-without-index/foo/foo.js b/fixtures/read-directory/children-directories-without-index/foo/foo.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-without-index/foo/foo.js
rename to fixtures/read-directory/children-directories-without-index/foo/foo.js
diff --git a/tests/fixtures/read-directory/children-directories-without-index/present.js b/fixtures/read-directory/children-directories-without-index/present.js
similarity index 100%
rename from tests/fixtures/read-directory/children-directories-without-index/present.js
rename to fixtures/read-directory/children-directories-without-index/present.js
diff --git a/tests/fixtures/read-directory/children-index/index.js b/fixtures/read-directory/children-directories/bar/index.js
similarity index 100%
rename from tests/fixtures/read-directory/children-index/index.js
rename to fixtures/read-directory/children-directories/bar/index.js
diff --git a/tests/fixtures/validate-target-directory/safe-index/index.js b/fixtures/read-directory/children-directories/foo/index.js
similarity index 100%
rename from tests/fixtures/validate-target-directory/safe-index/index.js
rename to fixtures/read-directory/children-directories/foo/index.js
diff --git a/tests/fixtures/read-directory/children-dot-files/present.js b/fixtures/read-directory/children-dot-files/present.js
similarity index 100%
rename from tests/fixtures/read-directory/children-dot-files/present.js
rename to fixtures/read-directory/children-dot-files/present.js
diff --git a/tests/fixtures/read-directory/children-files-multiple-extensions/bar.bar.js b/fixtures/read-directory/children-files-multiple-extensions/bar.bar.js
similarity index 100%
rename from tests/fixtures/read-directory/children-files-multiple-extensions/bar.bar.js
rename to fixtures/read-directory/children-files-multiple-extensions/bar.bar.js
diff --git a/tests/fixtures/read-directory/children-files-multiple-extensions/foo.foo.js b/fixtures/read-directory/children-files-multiple-extensions/foo.foo.js
similarity index 100%
rename from tests/fixtures/read-directory/children-files-multiple-extensions/foo.foo.js
rename to fixtures/read-directory/children-files-multiple-extensions/foo.foo.js
diff --git a/tests/fixtures/read-directory/children-files-multiple-extensions/present.js b/fixtures/read-directory/children-files-multiple-extensions/present.js
similarity index 100%
rename from tests/fixtures/read-directory/children-files-multiple-extensions/present.js
rename to fixtures/read-directory/children-files-multiple-extensions/present.js
diff --git a/tests/fixtures/read-directory/children-files-no-extension/bar b/fixtures/read-directory/children-files-no-extension/bar
similarity index 100%
rename from tests/fixtures/read-directory/children-files-no-extension/bar
rename to fixtures/read-directory/children-files-no-extension/bar
diff --git a/tests/fixtures/read-directory/children-files-no-extension/foo b/fixtures/read-directory/children-files-no-extension/foo
similarity index 100%
rename from tests/fixtures/read-directory/children-files-no-extension/foo
rename to fixtures/read-directory/children-files-no-extension/foo
diff --git a/tests/fixtures/read-directory/children-files-no-extension/present.js b/fixtures/read-directory/children-files-no-extension/present.js
similarity index 100%
rename from tests/fixtures/read-directory/children-files-no-extension/present.js
rename to fixtures/read-directory/children-files-no-extension/present.js
diff --git a/tests/fixtures/read-directory/children-files/bar.js b/fixtures/read-directory/children-files/bar.js
similarity index 100%
rename from tests/fixtures/read-directory/children-files/bar.js
rename to fixtures/read-directory/children-files/bar.js
diff --git a/tests/fixtures/read-directory/children-files/foo.js b/fixtures/read-directory/children-files/foo.js
similarity index 100%
rename from tests/fixtures/read-directory/children-files/foo.js
rename to fixtures/read-directory/children-files/foo.js
diff --git a/tests/fixtures/write-index/mixed/bar/index.js b/fixtures/read-directory/children-index/bar/index.js
similarity index 100%
rename from tests/fixtures/write-index/mixed/bar/index.js
rename to fixtures/read-directory/children-index/bar/index.js
diff --git a/tests/fixtures/write-index/mixed/foo/index.js b/fixtures/read-directory/children-index/foo/index.js
similarity index 100%
rename from tests/fixtures/write-index/mixed/foo/index.js
rename to fixtures/read-directory/children-index/foo/index.js
diff --git a/fixtures/read-directory/children-index/index.js b/fixtures/read-directory/children-index/index.js
new file mode 100644
index 0000000..a45ca72
--- /dev/null
+++ b/fixtures/read-directory/children-index/index.js
@@ -0,0 +1 @@
+'create index';
diff --git a/tests/fixtures/validate-target-directory/no-index/present.js b/fixtures/validate-target-directory/no-index/present.js
similarity index 100%
rename from tests/fixtures/validate-target-directory/no-index/present.js
rename to fixtures/validate-target-directory/no-index/present.js
diff --git a/tests/fixtures/validate-target-directory/not-a-directory.js b/fixtures/validate-target-directory/not-a-directory.js
similarity index 100%
rename from tests/fixtures/validate-target-directory/not-a-directory.js
rename to fixtures/validate-target-directory/not-a-directory.js
diff --git a/fixtures/validate-target-directory/safe-index/index.js b/fixtures/validate-target-directory/safe-index/index.js
new file mode 100644
index 0000000..a45ca72
--- /dev/null
+++ b/fixtures/validate-target-directory/safe-index/index.js
@@ -0,0 +1 @@
+'create index';
diff --git a/tests/fixtures/validate-target-directory/unsafe-index/index.js b/fixtures/validate-target-directory/unsafe-index/index.js
similarity index 100%
rename from tests/fixtures/validate-target-directory/unsafe-index/index.js
rename to fixtures/validate-target-directory/unsafe-index/index.js
diff --git a/fixtures/write-index/mixed/bar/index.js b/fixtures/write-index/mixed/bar/index.js
new file mode 100644
index 0000000..a45ca72
--- /dev/null
+++ b/fixtures/write-index/mixed/bar/index.js
@@ -0,0 +1 @@
+'create index';
diff --git a/tests/fixtures/write-index/mixed/foo.js b/fixtures/write-index/mixed/foo.js
similarity index 100%
rename from tests/fixtures/write-index/mixed/foo.js
rename to fixtures/write-index/mixed/foo.js
diff --git a/fixtures/write-index/mixed/foo/index.js b/fixtures/write-index/mixed/foo/index.js
new file mode 100644
index 0000000..a45ca72
--- /dev/null
+++ b/fixtures/write-index/mixed/foo/index.js
@@ -0,0 +1 @@
+'create index';
diff --git a/fixtures/write-index/mixed/index.js b/fixtures/write-index/mixed/index.js
new file mode 100644
index 0000000..1c830cb
--- /dev/null
+++ b/fixtures/write-index/mixed/index.js
@@ -0,0 +1,5 @@
+'create index';
+
+export bar from './bar';
+export foo from './foo.js';
+
diff --git a/package.json b/package.json
index 28d1db3..1c6ab8d 100644
--- a/package.json
+++ b/package.json
@@ -37,6 +37,7 @@
     "pragmatist": "^3.0.3"
   },
   "dependencies": {
+    "glob": "^6.0.4",
     "lodash": "^4.0.1",
     "yargs": "^4.0.0-alpha1"
   }
diff --git a/src/bin/create-index.js b/src/bin/create-index.js
index e02b411..1793604 100644
--- a/src/bin/create-index.js
+++ b/src/bin/create-index.js
@@ -9,6 +9,17 @@ let argv;
 
 argv = yargs
     .demand(1)
+    .options({
+        'update-index': {
+            default: false,
+            description: 'Recursively iterates target directories looking for "index.js" files that start with "\'create index\';\\n" (create-index index file). Updates found index files. Does not create new index files.',
+            type: 'boolean'
+        }
+    })
+    .example('create-index ./src ./src/utilities', 'Creates or updates an existing create-index index file in the target (./src, ./src/utilities) directories.')
+    .example('create-index --update-index ./src ./tests', 'Finds all create-index index files in the target directories and descending directories. Updates found index files.')
     .argv;
 
-writeIndexCli(argv._);
+writeIndexCli(argv._, {
+    updateIndex: argv.updateIndex
+});
diff --git a/src/index.js b/src/index.js
index c0f42c0..498c0a2 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,6 +1,4 @@
 export {
-    createIndexCode,
-    readDirectory,
-    validateTargetDirectory,
+    findIndexFiles,
     writeIndex
 } from './utilities';
diff --git a/src/utilities/findIndexFiles.js b/src/utilities/findIndexFiles.js
new file mode 100644
index 0000000..4e276c2
--- /dev/null
+++ b/src/utilities/findIndexFiles.js
@@ -0,0 +1,28 @@
+import _ from 'lodash';
+import glob from 'glob';
+import path from 'path';
+import validateTargetDirectory from './validateTargetDirectory';
+
+export default (directoryPath) => {
+    let targetDirectories;
+
+    targetDirectories = glob.sync(path.join(directoryPath, './**/index.js'));
+
+    targetDirectories = _.filter(targetDirectories, (targetDirectoryPath) => {
+        try {
+            validateTargetDirectory(path.dirname(targetDirectoryPath));
+
+            return true;
+        /* eslint-disable no-empty */
+        } catch (error) {
+
+        }
+        /* eslint-enable no-empty */
+
+        return false;
+    });
+
+    targetDirectories = _.map(targetDirectories, path.dirname);
+
+    return targetDirectories;
+};
diff --git a/src/utilities/index.js b/src/utilities/index.js
index 66da76d..ffbc3fa 100644
--- a/src/utilities/index.js
+++ b/src/utilities/index.js
@@ -1,6 +1,7 @@
 'create index';
 
 export createIndexCode from './createIndexCode.js';
+export findIndexFiles from './findIndexFiles.js';
 export log from './log.js';
 export readDirectory from './readDirectory.js';
 export sortByDepth from './sortByDepth.js';
diff --git a/src/utilities/writeIndexCli.js b/src/utilities/writeIndexCli.js
index ef5c566..b9df349 100644
--- a/src/utilities/writeIndexCli.js
+++ b/src/utilities/writeIndexCli.js
@@ -6,14 +6,25 @@ import validateTargetDirectory from './validateTargetDirectory';
 import readDirectory from './readDirectory';
 import sortByDepth from './sortByDepth';
 import log from './log';
+import findIndexFiles from './findIndexFiles';
 import chalk from 'chalk';
 
-export default (directoryPaths) => {
+export default (directoryPaths, options = {}) => {
     let sortedDirectoryPaths;
 
     sortedDirectoryPaths = sortByDepth(directoryPaths);
 
     log('Target directories', sortedDirectoryPaths);
+    log('Update index:', options.updateIndex ? chalk.green('true') : chalk.red('false'));
+
+    if (options.updateIndex) {
+        sortedDirectoryPaths = _.map(sortedDirectoryPaths, findIndexFiles);
+        sortedDirectoryPaths = _.flatten(sortedDirectoryPaths);
+        sortedDirectoryPaths = _.uniq(sortedDirectoryPaths);
+        sortedDirectoryPaths = sortByDepth(sortedDirectoryPaths);
+
+        log('Found index file in:', sortedDirectoryPaths);
+    }
 
     _.forEach(sortedDirectoryPaths, (directoryPath) => {
         validateTargetDirectory(directoryPath);
diff --git a/tests/findIndexFiles.js b/tests/findIndexFiles.js
new file mode 100644
index 0000000..56445aa
--- /dev/null
+++ b/tests/findIndexFiles.js
@@ -0,0 +1,20 @@
+import {
+    expect
+} from 'chai';
+
+import path from 'path';
+import glob from 'glob';
+import findIndexFiles from './../src/utilities/findIndexFiles';
+
+const fixturesPath = path.resolve(__dirname, './../../fixtures/find-index-files');
+
+describe('findIndexFiles()', () => {
+    it('finds only the directories that have an existing valid index file', () => {
+        let names;
+
+        names = findIndexFiles(path.resolve(fixturesPath));
+        names = names.sort();
+
+        expect(names).to.deep.equal(glob.sync(path.resolve(fixturesPath, './**/find-*')));
+    });
+});
diff --git a/tests/readDirectory.js b/tests/readDirectory.js
index ef60de9..4d05974 100644
--- a/tests/readDirectory.js
+++ b/tests/readDirectory.js
@@ -5,7 +5,7 @@ import {
 import path from 'path';
 import readDirectory from './../src/utilities/readDirectory';
 
-const fixturesPath = path.resolve(__dirname, './fixtures/read-directory');
+const fixturesPath = path.resolve(__dirname, './../../fixtures/read-directory');
 
 describe('readDirectory()', () => {
     context('target directory contains child directories', () => {
@@ -35,7 +35,7 @@ describe('readDirectory()', () => {
             expect(names).to.deep.equal(['present']);
         });
     });
-    context.skip('target directory contains ./index.js', () => {
+    context('target directory contains ./index.js', () => {
         it('does not include ./index.js', () => {
             let names;
 
diff --git a/tests/validateTargetDirectory.js b/tests/validateTargetDirectory.js
index 427615a..c0687fe 100644
--- a/tests/validateTargetDirectory.js
+++ b/tests/validateTargetDirectory.js
@@ -6,7 +6,7 @@ import {
 import path from 'path';
 import validateTargetDirectory from './../src/utilities/validateTargetDirectory';
 
-const fixturesPath = path.resolve(__dirname, './fixtures/validate-target-directory');
+const fixturesPath = path.resolve(__dirname, './../../fixtures/validate-target-directory');
 
 describe('validateTargetDirectory()', () => {
     describe('directory path', () => {
@@ -31,7 +31,7 @@ describe('validateTargetDirectory()', () => {
                 expect(validateTargetDirectory(path.resolve(fixturesPath, 'no-index'))).to.equal(true);
             });
         });
-        context.skip('safe', () => {
+        context('safe', () => {
             it('returns true', () => {
                 expect(validateTargetDirectory(path.resolve(fixturesPath, 'safe-index'))).to.equal(true);
             });
diff --git a/tests/writeIndex.js b/tests/writeIndex.js
index 825a850..ffd48fb 100644
--- a/tests/writeIndex.js
+++ b/tests/writeIndex.js
@@ -8,7 +8,7 @@ import path from 'path';
 import codeExample from './codeExample';
 import writeIndex from './../src/utilities/writeIndex';
 
-const fixturesPath = path.resolve(__dirname, './fixtures/write-index');
+const fixturesPath = path.resolve(__dirname, './../../fixtures/write-index');
 
 describe('writeIndex()', () => {
     it('creates index in target directory', () => {