diff --git a/packages/serverless-runtime-types/types.d.ts b/packages/serverless-runtime-types/types.d.ts
index 92479828..4f9ac6a8 100644
--- a/packages/serverless-runtime-types/types.d.ts
+++ b/packages/serverless-runtime-types/types.d.ts
@@ -1,6 +1,10 @@
 import twilio from 'twilio';
 import { ServiceContext } from 'twilio/lib/rest/sync/v1/service';
 
+export type EnvironmentVariables = {
+  [key: string]: string | undefined;
+};
+
 export type ResourceMap = {
   [name: string]: {
     path: string;
@@ -17,3 +21,17 @@ export type Context<T = {}> = {
   getTwilioClient(): twilio.Twilio;
   DOMAIN_NAME: string;
 } & T;
+
+export type ServerlessCallback = (
+  error: null | Error,
+  payload?: object
+) => void;
+
+export type ServerlessFunctionSignature<
+  T extends EnvironmentVariables = {},
+  U extends {} = {}
+> = (
+  context: Context<T>,
+  event: U,
+  callback: ServerlessCallback
+) => void | Promise<void>;