diff --git a/functions/firebase/.gcloudignore b/functions/firebase/.gcloudignore
new file mode 100644
index 0000000000..ccc4eb240e
--- /dev/null
+++ b/functions/firebase/.gcloudignore
@@ -0,0 +1,16 @@
+# This file specifies files that are *not* uploaded to Google Cloud Platform
+# using gcloud. It follows the same syntax as .gitignore, with the addition of
+# "#!include" directives (which insert the entries of the given .gitignore-style
+# file at that point).
+#
+# For more information, run:
+#   $ gcloud topic gcloudignore
+#
+.gcloudignore
+# If you would like to upload your .git directory, .gitignore file or files
+# from your .gitignore file, remove the corresponding line
+# below:
+.git
+.gitignore
+
+node_modules
diff --git a/functions/firebase/index.js b/functions/firebase/index.js
index 15a158c039..96adf500fa 100644
--- a/functions/firebase/index.js
+++ b/functions/firebase/index.js
@@ -84,3 +84,25 @@ exports.helloAuth = (event, callback) => {
   callback();
 };
 // [END functions_firebase_auth]
+
+// [START functions_firebase_reactive]
+const Firestore = require('@google-cloud/firestore');
+
+const firestore = new Firestore({
+  projectId: process.env.GCP_PROJECT
+});
+
+// Converts strings added to /messages/{pushId}/original to uppercase
+exports.makeUpperCase = (event, callback) => {
+  const resource = event.resource;
+  const affectedDoc = firestore.doc(resource.split('/documents/')[1]);
+
+  const curValue = event.data.value.fields.original.stringValue;
+  const newValue = curValue.toUpperCase();
+  console.log(`Replacing value: ${curValue} --> ${newValue}`);
+
+  return affectedDoc.set({
+    'original': newValue
+  });
+};
+// [END functions_firebase_reactive]
diff --git a/functions/firebase/package.json b/functions/firebase/package.json
new file mode 100644
index 0000000000..eb97e5d156
--- /dev/null
+++ b/functions/firebase/package.json
@@ -0,0 +1,30 @@
+{
+  "name": "nodejs-docs-samples-functions-firebase",
+  "version": "0.0.1",
+  "private": true,
+  "license": "Apache-2.0",
+  "author": "Google Inc.",
+  "repository": {
+    "type": "git",
+    "url": "https://github.com/GoogleCloudPlatform/nodejs-docs-samples.git"
+  },
+  "engines": {
+    "node": ">=4.3.2"
+  },
+  "scripts": {
+    "pretest": "repo-tools lint",
+    "test": "ava -T 30s test/*.test.js"
+  },
+  "devDependencies": {
+    "@google-cloud/nodejs-repo-tools": "^2.2.5",
+    "ava": "0.25.0",
+    "proxyquire": "2.0.1",
+    "semistandard": "^12.0.1",
+    "sinon": "4.4.8",
+    "supertest": "^3.0.0",
+    "uuid": "^3.1.0"
+  },
+  "dependencies": {
+    "@google-cloud/firestore": "^0.17.0"
+  }
+}
diff --git a/functions/firebase/test/index.test.js b/functions/firebase/test/index.test.js
new file mode 100644
index 0000000000..c085331eb3
--- /dev/null
+++ b/functions/firebase/test/index.test.js
@@ -0,0 +1,143 @@
+/**
+ * Copyright 2018, Google LLC.
+ * 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.
+ */
+
+'use strict';
+
+const proxyquire = require(`proxyquire`).noCallThru();
+const sinon = require(`sinon`);
+const test = require(`ava`);
+const tools = require(`@google-cloud/nodejs-repo-tools`);
+
+function getSample () {
+  const firestoreMock = {
+    doc: sinon.stub().returnsThis(),
+    set: sinon.stub()
+  };
+
+  return {
+    program: proxyquire(`../`, {
+      '@google-cloud/firestore': sinon.stub().returns(firestoreMock)
+    }),
+    mocks: {
+      firestore: firestoreMock
+    }
+  };
+}
+
+test.beforeEach(tools.stubConsole);
+test.afterEach.always(tools.restoreConsole);
+
+test(`should listen to RTDB`, t => {
+  const sample = getSample();
+
+  const delta = {
+    foo: 'bar'
+  };
+  const event = {
+    resource: 'resource',
+    auth: {
+      admin: true
+    },
+    delta: delta
+  };
+  const cb = sinon.stub();
+
+  sample.program.helloRTDB(event, cb);
+
+  t.true(console.log.calledWith(`Function triggered by change to: resource`));
+  t.true(console.log.calledWith(`Admin?: true`));
+  t.true(console.log.calledWith(JSON.stringify(delta, null, 2)));
+  t.true(cb.calledOnce);
+});
+
+test(`should listen to Firestore`, t => {
+  const sample = getSample();
+
+  const oldValue = {
+    foo: 'bar'
+  };
+  const value = {
+    bar: 'baz'
+  };
+  const event = {
+    resource: 'resource',
+    eventType: 'type',
+    data: {
+      oldValue: oldValue,
+      value: value
+    }
+  };
+  const cb = sinon.stub();
+
+  sample.program.helloFirestore(event, cb);
+
+  t.true(console.log.calledWith(`Function triggered by event on: resource`));
+  t.true(console.log.calledWith(`Event type: type`));
+  t.true(console.log.calledWith(JSON.stringify(oldValue, null, 2)));
+  t.true(console.log.calledWith(JSON.stringify(value, null, 2)));
+  t.true(cb.calledOnce);
+});
+
+test(`should listen to Auth events`, t => {
+  const sample = getSample();
+  const date = Date.now();
+  const event = {
+    resource: 'resource',
+    data: {
+      uid: 'me',
+      email: 'me@example.com',
+      metadata: {
+        createdAt: date
+      }
+    }
+  };
+  const cb = sinon.stub();
+
+  sample.program.helloAuth(event, cb);
+
+  t.true(console.log.calledWith(`Function triggered by change to user: me`));
+  t.true(console.log.calledWith(`Created at: ${date}`));
+  t.true(console.log.calledWith(`Email: me@example.com`));
+  t.true(cb.calledOnce);
+});
+
+test(`should update data in response to Firestore events`, t => {
+  const sample = getSample();
+
+  const date = Date.now();
+  const event = {
+    resource: '/documents/some/path',
+    data: {
+      email: 'me@example.com',
+      metadata: {
+        createdAt: date
+      },
+      value: {
+        fields: {
+          original: {
+            stringValue: 'foobar'
+          }
+        }
+      }
+    }
+  };
+  const cb = sinon.stub();
+
+  sample.program.makeUpperCase(event, cb);
+
+  t.true(sample.mocks.firestore.doc.calledWith('some/path'));
+  t.true(console.log.calledWith(`Replacing value: foobar --> FOOBAR`));
+  t.true(sample.mocks.firestore.set.calledWith({'original': 'FOOBAR'}));
+});