diff --git a/lib/modules/admob/index.js b/lib/modules/admob/index.js
index 2ca393cb..c044646b 100644
--- a/lib/modules/admob/index.js
+++ b/lib/modules/admob/index.js
@@ -16,11 +16,22 @@ import EventTypes, {
   RewardedVideoEventTypes,
 } from './EventTypes';
 
+import type FirebaseApp from '../core/firebase-app';
+
+type NativeEvent = {
+  adUnit: string,
+  payload: Object,
+  type: string,
+}
+
 export default class AdMob extends ModuleBase {
   static _NAMESPACE = 'admob';
   static _NATIVE_MODULE = 'RNFirebaseAdMob';
 
-  constructor(firebaseApp: Object, options: Object = {}) {
+  _appId: ?string;
+  _initialized: boolean;
+
+  constructor(firebaseApp: FirebaseApp, options: Object = {}) {
     super(firebaseApp, options, true);
 
     this._initialized = false;
@@ -30,7 +41,7 @@ export default class AdMob extends ModuleBase {
     this._eventEmitter.addListener('rewarded_video_event', this._onRewardedVideoEvent.bind(this));
   }
 
-  _onInterstitialEvent(event) {
+  _onInterstitialEvent(event: NativeEvent): void {
     const { adUnit } = event;
     const jsEventType = `interstitial_${adUnit}`;
 
@@ -41,7 +52,7 @@ export default class AdMob extends ModuleBase {
     this.emit(jsEventType, event);
   }
 
-  _onRewardedVideoEvent(event) {
+  _onRewardedVideoEvent(event: NativeEvent): void {
     const { adUnit } = event;
     const jsEventType = `rewarded_video_${adUnit}`;
 
@@ -52,7 +63,7 @@ export default class AdMob extends ModuleBase {
     this.emit(jsEventType, event);
   }
 
-  initialize(appId: string) {
+  initialize(appId: string): void {
     if (this._initialized) {
       this.log.warn('AdMob has already been initialized!');
     } else {
@@ -62,7 +73,7 @@ export default class AdMob extends ModuleBase {
     }
   }
 
-  openDebugMenu() {
+  openDebugMenu(): void {
     if (!this._initialized) {
       this.log.warn('AdMob needs to be initialized before opening the dev menu!');
     } else {
@@ -71,11 +82,11 @@ export default class AdMob extends ModuleBase {
     }
   }
 
-  interstitial(adUnit: string) {
+  interstitial(adUnit: string): Interstitial {
     return new Interstitial(this, adUnit);
   }
 
-  rewarded(adUnit: string) {
+  rewarded(adUnit: string): RewardedVideo {
     return new RewardedVideo(this, adUnit);
   }