diff --git a/source/migration-helper.ts b/source/migration-helper.ts
index fb9df3a..61bc7a2 100644
--- a/source/migration-helper.ts
+++ b/source/migration-helper.ts
@@ -7,9 +7,9 @@
  * The migrate function should take the data you pass through {@link migrate}
  * and modify it to your needs.
  */
-export type Migration<V> = {
+export type Migration<V, A = any, B = any> = {
   version: V;
-  migrate: (data: any) => Promise<unknown>;
+  migrate: (data: A) => Promise<B>;
 };
 
 /**
diff --git a/tests/date.test.ts b/tests/date.test.ts
index f4c0b71..0e53c79 100644
--- a/tests/date.test.ts
+++ b/tests/date.test.ts
@@ -3,7 +3,7 @@ import test from 'ava';
 import {migrate, Migration} from '../source/migration-helper.js';
 
 test('migrate<Date>', async (t) => {
-  const migrations: Array<Migration<Date>> = [
+  const migrations: Array<Migration<Date, string, string>> = [
     {
       version: new Date('2022-01-01'),
       migrate: async (data: string) => `${data} migrated`,
diff --git a/tests/number.test.ts b/tests/number.test.ts
index 209a2d9..dcf14d4 100644
--- a/tests/number.test.ts
+++ b/tests/number.test.ts
@@ -3,7 +3,7 @@ import test from 'ava';
 import {migrate, Migration} from '../source/migration-helper.js';
 
 test('migrate<number>', async (t) => {
-  const migrations: Array<Migration<number>> = [
+  const migrations: Array<Migration<number, string, string>> = [
     {
       version: 5,
       migrate: async (data: string) => `${data} migrated`,
diff --git a/tests/object.test.ts b/tests/object.test.ts
index 513590a..55a9475 100644
--- a/tests/object.test.ts
+++ b/tests/object.test.ts
@@ -12,7 +12,7 @@ test('migrate<{custom: number}>', async (t) => {
   };
 
   const skip: SkipMigrationFn<Custom> = (a, b) => a.custom > b.custom;
-  const migrations: Array<Migration<Custom>> = [
+  const migrations: Array<Migration<Custom, string, string>> = [
     {
       version: {custom: 5},
       migrate: async (data: string) => `${data} migrated`,
diff --git a/tests/string.test.ts b/tests/string.test.ts
index 49e446a..f800309 100644
--- a/tests/string.test.ts
+++ b/tests/string.test.ts
@@ -3,7 +3,7 @@ import test from 'ava';
 import {migrate, Migration} from '../source/migration-helper.js';
 
 test('migrate<string>', async (t) => {
-  const migrations: Array<Migration<string>> = [
+  const migrations: Array<Migration<string, string, string>> = [
     {
       version: '0.2.4',
       migrate: async (data: string) => `${data} migrated`,