diff --git a/package.json b/package.json
index ecdc5042a..8e84bd7bc 100644
--- a/package.json
+++ b/package.json
@@ -101,6 +101,7 @@
     "@google-cloud/nodejs-repo-tools": "^2.3.3",
     "@types/arrify": "^1.0.4",
     "@types/async": "^2.0.50",
+    "@types/duplexify": "^3.6.0",
     "@types/extend": "^3.0.0",
     "@types/is": "0.0.20",
     "@types/mocha": "^5.2.5",
diff --git a/src/connection-pool.ts b/src/connection-pool.ts
index 471d96d72..a8bfa986a 100644
--- a/src/connection-pool.ts
+++ b/src/connection-pool.ts
@@ -18,10 +18,14 @@ import {replaceProjectIdToken} from '@google-cloud/projectify';
 const duplexify = require('duplexify');
 const each = require('async-each');
 import {EventEmitter} from 'events';
-import * as is from 'is';
 import * as through from 'through2';
 import * as uuid from 'uuid';
 import * as util from './util';
+import { Subscription } from './subscription';
+import { PubSub } from '.';
+import { Duplex } from 'stream';
+import { StatusObject } from 'grpc';
+import { Subscriber } from './subscriber';
 
 const CHANNEL_READY_EVENT = 'channel.ready';
 const CHANNEL_ERROR_EVENT = 'channel.error';
@@ -49,6 +53,16 @@ const RETRY_CODES = [
   15, // dataloss
 ];
 
+export interface ConnectionPoolSettings {
+  maxConnections: number;
+  ackDeadline: number;
+}
+
+export type ConnectionResponse = [Duplex];
+export interface ConnectionCallback {
+  (err: Error|null, connection?: Duplex): void;
+}
+
 /*!
  * ConnectionPool is used to manage the stream connections created via
  * StreamingPull rpc.
@@ -62,19 +76,19 @@ const RETRY_CODES = [
  *     creating a connection.
  */
 export class ConnectionPool extends EventEmitter {
-  subscription;
-  pubsub;
-  connections;
-  isPaused;
-  isOpen;
-  isGettingChannelState;
-  failedConnectionAttempts;
-  noConnectionsTime;
-  settings;
-  queue;
-  keepAliveHandle;
-  client;
-  constructor(subscription) {
+  subscription: Subscription;
+  pubsub: PubSub;
+  connections: Map<string, Duplex>;
+  isPaused: boolean;
+  isOpen: boolean;
+  isGettingChannelState: boolean;
+  failedConnectionAttempts: number;
+  noConnectionsTime: number;
+  settings: ConnectionPoolSettings;
+  queue: NodeJS.Timer[];
+  keepAliveHandle?: NodeJS.Timer;
+  client?: Subscriber|null;
+  constructor(subscription: Subscription) {
     super();
     this.subscription = subscription;
     this.pubsub = subscription.pubsub;
@@ -103,21 +117,23 @@ export class ConnectionPool extends EventEmitter {
    *     connection.
    * @param {stream} callback.connection A duplex stream.
    */
-  acquire(id, callback) {
-    if (is.fn(id)) {
-      callback = id;
-      id = null;
-    }
+  acquire(id?: string): Promise<ConnectionResponse>;
+  acquire(id: string, callback: ConnectionCallback): void;
+  acquire(callback: ConnectionCallback): void;
+  acquire(idOrCallback?: string|ConnectionCallback, cb?: ConnectionCallback): void|Promise<ConnectionResponse> {
+    let id = typeof idOrCallback === 'string' ? idOrCallback : null;
+    const callback = typeof idOrCallback === 'function' ? idOrCallback : cb!;
+
     if (!this.isOpen) {
       callback(new Error('No connections available to make request.'));
       return;
     }
     // it's possible that by the time a user acks the connection could have
     // closed, so in that case we'll just return any connection
-    if (!this.connections.has(id)) {
+    if (!this.connections.has(id!)) {
       id = this.connections.keys().next().value;
     }
-    const connection = this.connections.get(id);
+    const connection = this.connections.get(id!);
     if (connection) {
       callback(null, connection);
       return;
@@ -137,7 +153,7 @@ export class ConnectionPool extends EventEmitter {
   close(callback) {
     const connections = Array.from(this.connections.values());
     callback = callback || util.noop;
-    clearInterval(this.keepAliveHandle);
+    clearInterval(this.keepAliveHandle!);
     this.connections.clear();
     this.queue.forEach(clearTimeout);
     this.queue.length = 0;
@@ -345,7 +361,8 @@ export class ConnectionPool extends EventEmitter {
     const interator = this.connections.values();
     let connection = interator.next().value;
     while (connection) {
-      if (connection.isConnected) {
+      // tslint:disable-next-line no-any
+      if ((connection as any).isConnected) {
         return true;
       }
       connection = interator.next().value;
@@ -416,7 +433,7 @@ export class ConnectionPool extends EventEmitter {
    *
    * @private
    */
-  resume() {
+  resume(): void {
     this.isPaused = false;
     this.connections.forEach(connection => {
       connection.resume();
@@ -427,7 +444,7 @@ export class ConnectionPool extends EventEmitter {
    *
    * @private
    */
-  sendKeepAlives() {
+  sendKeepAlives(): void {
     this.connections.forEach(connection => {
       connection.write({});
     });
@@ -440,7 +457,7 @@ export class ConnectionPool extends EventEmitter {
    * @param {object} status The gRPC status object.
    * @return {boolean}
    */
-  shouldReconnect(status) {
+  shouldReconnect(status: StatusObject): boolean {
     // If the pool was closed, we should definitely not reconnect
     if (!this.isOpen) {
       return false;
diff --git a/src/histogram.ts b/src/histogram.ts
index 6a058a25a..8c25285a9 100644
--- a/src/histogram.ts
+++ b/src/histogram.ts
@@ -16,6 +16,11 @@
 
 import * as extend from 'extend';
 
+export interface HistogramOptions {
+  min?: number;
+  max?: number;
+}
+
 /*!
  * The Histogram class is used to capture the lifespan of messages within the
  * the client. These durations are then used to calculate the 99th percentile
@@ -25,10 +30,10 @@ import * as extend from 'extend';
  * @class
  */
 export class Histogram {
-  options;
-  data;
-  length;
-  constructor(options?) {
+  options: HistogramOptions;
+  data: Map<number, number>;
+  length: number;
+  constructor(options?: HistogramOptions) {
     this.options = extend(
       {
         min: 10000,
@@ -45,14 +50,14 @@ export class Histogram {
    * @private
    * @param {numnber} value - The value in milliseconds.
    */
-  add(value) {
-    value = Math.max(value, this.options.min);
-    value = Math.min(value, this.options.max);
+  add(value: number): void {
+    value = Math.max(value, this.options.min!);
+    value = Math.min(value, this.options.max!);
     value = Math.ceil(value / 1000) * 1000;
     if (!this.data.has(value)) {
       this.data.set(value, 0);
     }
-    const count = this.data.get(value);
+    const count = this.data.get(value)!;
     this.data.set(value, count + 1);
     this.length += 1;
   }
@@ -63,18 +68,18 @@ export class Histogram {
    * @param {number} percent The requested percentage.
    * @return {number}
    */
-  percentile(percent) {
+  percentile(percent: number): number {
     percent = Math.min(percent, 100);
     let target = this.length - this.length * (percent / 100);
     const keys = Array.from(this.data.keys());
     let key;
     for (let i = keys.length - 1; i > -1; i--) {
       key = keys[i];
-      target -= this.data.get(key);
+      target -= this.data.get(key)!;
       if (target <= 0) {
         return key;
       }
     }
-    return this.options.min;
+    return this.options.min!;
   }
 }
diff --git a/src/iam.ts b/src/iam.ts
index 2c74842b0..0047acdc8 100644
--- a/src/iam.ts
+++ b/src/iam.ts
@@ -21,6 +21,7 @@
 import * as arrify from 'arrify';
 import {promisifyAll} from '@google-cloud/promisify';
 import * as is from 'is';
+import { PubSub } from '.';
 
 /**
  * [IAM (Identity and Access Management)](https://cloud.google.com/pubsub/access_control)
@@ -61,11 +62,11 @@ import * as is from 'is';
  * // subscription.iam
  */
 export class IAM {
-  Promise;
-  pubsub;
-  request;
-  id;
-  constructor(pubsub, id) {
+  Promise?: PromiseConstructor;
+  pubsub: PubSub;
+  request: typeof PubSub.prototype.request;
+  id: string;
+  constructor(pubsub: PubSub, id: string) {
     if (pubsub.Promise) {
       this.Promise = pubsub.Promise;
     }
@@ -282,7 +283,7 @@ export class IAM {
    *   const apiResponse = data[1];
    * });
    */
-  testPermissions(permissions, gaxOpts, callback?) {
+  testPermissions(permissions: string|string[], gaxOpts, callback?) {
     if (!is.array(permissions) && !is.string(permissions)) {
       throw new Error('Permissions are required.');
     }
@@ -307,7 +308,7 @@ export class IAM {
           return;
         }
         const availablePermissions = arrify(resp.permissions);
-        const permissionHash = permissions.reduce(function(acc, permission) {
+        const permissionHash = (permissions as string[]).reduce(function(acc, permission) {
           acc[permission] = availablePermissions.indexOf(permission) > -1;
           return acc;
         }, {});
diff --git a/src/index.ts b/src/index.ts
index 579f3f6ce..2eb951647 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -19,8 +19,7 @@ import {paginator} from '@google-cloud/paginator';
 import {promisifyAll} from '@google-cloud/promisify';
 import * as extend from 'extend';
 import {GoogleAuth} from 'google-auth-library';
-const gax = require('google-gax');
-const {grpc} = new gax.GrpcClient();
+import * as gax from 'google-gax';
 import * as is from 'is';
 
 const PKG = require('../../package.json');
@@ -31,6 +30,9 @@ import {Subscription} from './subscription';
 import {Topic} from './topic';
 import { Readable } from 'stream';
 
+const opts = {} as gax.GrpcClientOptions;
+const {grpc} = new gax.GrpcClient(opts);
+
 /**
  * @type {string} - Project ID placeholder.
  * @private
@@ -206,14 +208,14 @@ export class PubSub {
    *   const apiResponse = data[1];
    * });
    */
-  createSubscription(topic, name, options, callback) {
+  createSubscription(topic: Topic|string, name: string, options, callback) {
     if (!is.string(topic) && !(topic instanceof Topic)) {
       throw new Error('A Topic is required for a new subscription.');
     }
     if (!is.string(name)) {
       throw new Error('A subscription name is required.');
     }
-    if (is.string(topic)) {
+    if (typeof topic === 'string') {
       topic = this.topic(topic);
     }
     if (is.fn(options)) {
@@ -286,7 +288,7 @@ export class PubSub {
    *   const apiResponse = data[1];
    * });
    */
-  createTopic(name, gaxOpts, callback?) {
+  createTopic(name: string, gaxOpts, callback?) {
     const topic = this.topic(name);
     const reqOpts = {
       name: topic.name,
@@ -691,7 +693,7 @@ export class PubSub {
    *
    * const snapshot = pubsub.snapshot('my-snapshot');
    */
-  snapshot(name) {
+  snapshot(name: string) {
     if (!is.string(name)) {
       throw new Error('You must supply a valid name for the snapshot.');
     }
diff --git a/src/publisher.ts b/src/publisher.ts
index abc62ca2b..5a9e5db11 100644
--- a/src/publisher.ts
+++ b/src/publisher.ts
@@ -48,8 +48,8 @@ import { Topic } from './topic';
  * const publisher = topic.publisher();
  */
 export class Publisher {
-  Promise;
-  topic;
+  Promise?: PromiseConstructor;
+  topic: Topic;
   inventory_;
   settings;
   timeoutHandle_;
@@ -148,7 +148,7 @@ export class Publisher {
    * //-
    * publisher.publish(data).then((messageId) => {});
    */
-  publish(data, attributes, callback?) {
+  publish(data: Buffer, attributes, callback?) {
     if (!(data instanceof Buffer)) {
       throw new TypeError('Data must be in the form of a Buffer.');
     }
diff --git a/src/snapshot.ts b/src/snapshot.ts
index 5e46116c2..0434da106 100644
--- a/src/snapshot.ts
+++ b/src/snapshot.ts
@@ -17,6 +17,7 @@
 import * as util from './util';
 import {promisifyAll} from '@google-cloud/promisify';
 import * as is from 'is';
+import { PubSub } from '.';
 
 /**
  * A Snapshot object will give you access to your Cloud Pub/Sub snapshot.
@@ -84,12 +85,12 @@ import * as is from 'is';
  */
 export class Snapshot {
   parent;
-  name;
-  Promise;
+  name: string;
+  Promise?: PromiseConstructor;
   create;
   seek;
   metadata;
-  constructor(parent, name) {
+  constructor(parent, name: string) {
     if (parent.Promise) {
       this.Promise = parent.Promise;
     }
@@ -201,7 +202,7 @@ export class Snapshot {
    *
    * @private
    */
-  static formatName_(projectId, name) {
+  static formatName_(projectId: string, name: string) {
     return 'projects/' + projectId + '/snapshots/' + name.split('/').pop();
   }
 }
diff --git a/src/subscriber.ts b/src/subscriber.ts
index 43d986298..5ea305776 100644
--- a/src/subscriber.ts
+++ b/src/subscriber.ts
@@ -26,6 +26,7 @@ import * as os from 'os';
 
 import {ConnectionPool} from './connection-pool';
 import {Histogram} from './histogram';
+import { Subscription } from '.';
 
 /**
  * @type {number} - The maximum number of ackIds to be sent in acknowledge/modifyAckDeadline
@@ -389,7 +390,8 @@ export class Subscriber extends EventEmitter {
    * @private
    */
   openConnection_() {
-    const pool = (this.connectionPool = new ConnectionPool(this));
+    // TODO: fixup this cast
+    const pool = (this.connectionPool = new ConnectionPool(this as {} as Subscription));
     this.isOpen = true;
     pool.on('error', err => {
       this.emit('error', err);
@@ -477,7 +479,7 @@ export class Subscriber extends EventEmitter {
         }
         // we can ignore any errors that come from this since they'll be
         // re-emitted later
-        connection.write(data, err => {
+        connection!.write(data, err => {
           if (!err) {
             this.latency_.add(Date.now() - startTime);
           }
diff --git a/src/subscription.ts b/src/subscription.ts
index 8fb23eace..563f16688 100644
--- a/src/subscription.ts
+++ b/src/subscription.ts
@@ -146,7 +146,7 @@ export class Subscription extends Subscriber {
   create!: Function;
   iam: IAM;
   metadata;
-  constructor(pubsub, name, options) {
+  constructor(pubsub: PubSub, name: string, options) {
     options = options || {};
     super(options);
     if (pubsub.Promise) {
@@ -686,7 +686,7 @@ export class Subscription extends Subscriber {
    * @example
    * const snapshot = subscription.snapshot('my-snapshot');
    */
-  snapshot(name) {
+  snapshot(name: string) {
     return this.pubsub.snapshot.call(this, name);
   }
   /*!
@@ -717,7 +717,7 @@ export class Subscription extends Subscriber {
    *
    * @private
    */
-  static formatName_(projectId, name) {
+  static formatName_(projectId: string, name: string) {
     // Simple check if the name is already formatted.
     if (name.indexOf('/') > -1) {
       return name;
diff --git a/src/topic.ts b/src/topic.ts
index 973022190..f2e060a3b 100644
--- a/src/topic.ts
+++ b/src/topic.ts
@@ -41,9 +41,9 @@ import { Readable } from 'stream';
 export class Topic {
   Promise?: PromiseConstructor;
   name: string;
-  parent;
+  parent: PubSub;
   pubsub: PubSub;
-  request;
+  request: typeof PubSub.prototype.request;
   iam: IAM;
   metadata;
   getSubscriptionsStream = paginator.streamify('getSubscriptions') as () => Readable;
@@ -178,7 +178,7 @@ export class Topic {
    *   const apiResponse = data[1];
    * });
    */
-  createSubscription(name, options, callback?) {
+  createSubscription(name: string, options, callback?) {
     this.pubsub.createSubscription(this, name, options, callback);
   }
   /**
diff --git a/system-test/pubsub.ts b/system-test/pubsub.ts
index b28d458c3..83b1000fa 100644
--- a/system-test/pubsub.ts
+++ b/system-test/pubsub.ts
@@ -17,7 +17,7 @@
 import * as assert from 'assert';
 import * as async from 'async';
 import * as uuid from 'uuid';
-import {PubSub, Subscription} from '../src';
+import {PubSub, Subscription, Topic} from '../src';
 
 const pubsub = new PubSub();
 
@@ -48,7 +48,7 @@ describe('pubsub', function() {
     return 'test-topic-' + uuid.v4();
   }
 
-  function getTopicName(topic) {
+  function getTopicName(topic: Topic) {
     return topic.name.split('/').pop();
   }