diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index b10eecf..81c79ae 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -19,7 +19,7 @@ jobs:
           - ubuntu-latest
           - macos-latest
           - windows-latest
-        node-version: [20.x, 22.x, 24.x]
+        node-version: [22.x, 24.x, 26.x]
       fail-fast: false
 
     steps:
@@ -44,7 +44,7 @@ jobs:
           persist-credentials: false
       - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
         with:
-          node-version: 24.x
+          node-version: 22.x
       - run: npm ci
       - run: npm run build
       - run: npm run coverage:check
@@ -58,7 +58,7 @@ jobs:
           persist-credentials: false
       - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
         with:
-          node-version: 24.x
+          node-version: 22.x
       - run: npm ci
       - run: npm run build
       - run: npm run exports:check
@@ -72,6 +72,6 @@ jobs:
           persist-credentials: false
       - uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0
         with:
-          node-version: 24.x
+          node-version: 22.x
       - run: npm ci
       - run: npm run docs
diff --git a/.mocharc.json b/.mocharc.json
index 40ba7d1..b5b7170 100644
--- a/.mocharc.json
+++ b/.mocharc.json
@@ -1,5 +1,5 @@
 {
-  "require": "ts-node/register",
+  "node-option": ["loader=ts-node/esm"],
   "extension": ["ts"],
   "recursive": true
 }
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9a67f7e..dd1f583 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Changed
+- BREAKING CHANGE: Switch to ESM ([#70](https://github.com/cucumber/javascript-core/pull/70))
 
 ## [0.8.3] - 2026-01-27
 ### Changed
diff --git a/package.json b/package.json
index ac75482..39a03ae 100644
--- a/package.json
+++ b/package.json
@@ -2,7 +2,7 @@
   "name": "@cucumber/core",
   "version": "0.8.3",
   "description": "Core test case composition library for building a Cucumber implementation",
-  "type": "commonjs",
+  "type": "module",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",
   "files": [
@@ -17,6 +17,7 @@
     "exports:update": "api-extractor run --verbose --local",
     "fix": "biome check --fix --error-on-warnings",
     "lint": "biome check --error-on-warnings",
+    "postbuild": "node -e \"require('.')\"",
     "test": "mocha \"src/**/*.spec.ts\"",
     "prepublishOnly": "npm run build"
   },
diff --git a/src/AmbiguousError.spec.ts b/src/AmbiguousError.spec.ts
index 1cf5050..919916b 100644
--- a/src/AmbiguousError.spec.ts
+++ b/src/AmbiguousError.spec.ts
@@ -2,8 +2,8 @@ import type { PickleStep, SourceReference } from '@cucumber/messages'
 import { expect } from 'chai'
 import sinon from 'sinon'
 
-import { AmbiguousError } from './AmbiguousError'
-import type { AmbiguousStep, DefinedStep } from './types'
+import { AmbiguousError } from './AmbiguousError.js'
+import type { AmbiguousStep, DefinedStep } from './types.js'
 
 describe('AmbiguousError', () => {
   it('handles source references with and without locations', () => {
diff --git a/src/AmbiguousError.ts b/src/AmbiguousError.ts
index 716738f..a9c0065 100644
--- a/src/AmbiguousError.ts
+++ b/src/AmbiguousError.ts
@@ -1,4 +1,4 @@
-import type { AmbiguousStep } from './types'
+import type { AmbiguousStep } from './types.js'
 
 /**
  * Represents an error that occurs when multiple step definitions are found matching the text of a step
diff --git a/src/DataTable.spec.ts b/src/DataTable.spec.ts
index 71d4b71..6c2634a 100644
--- a/src/DataTable.spec.ts
+++ b/src/DataTable.spec.ts
@@ -2,8 +2,8 @@ import type { PickleTable } from '@cucumber/messages'
 import { expect } from 'chai'
 import { describe, it } from 'mocha'
 
-import { parseGherkin } from '../test/parseGherkin'
-import { DataTable } from './DataTable'
+import { parseGherkin } from '../test/parseGherkin.js'
+import { DataTable } from './DataTable.js'
 
 describe('DataTable', () => {
   describe('raw', () => {
diff --git a/src/SupportCodeBuilderImpl.ts b/src/SupportCodeBuilderImpl.ts
index b543459..65c07a1 100644
--- a/src/SupportCodeBuilderImpl.ts
+++ b/src/SupportCodeBuilderImpl.ts
@@ -7,7 +7,7 @@ import {
 import { HookType, StepDefinitionPatternType } from '@cucumber/messages'
 import parse from '@cucumber/tag-expressions'
 
-import { SupportCodeLibraryImpl } from './SupportCodeLibraryImpl'
+import { SupportCodeLibraryImpl } from './SupportCodeLibraryImpl.js'
 import type {
   DefinedParameterType,
   DefinedStep,
@@ -19,7 +19,7 @@ import type {
   NewTestRunHook,
   SupportCodeBuilder,
   UndefinedParameterType,
-} from './types'
+} from './types.js'
 
 type Registered<T> = { id: string; order: number } & T
 
diff --git a/src/SupportCodeLibraryImpl.ts b/src/SupportCodeLibraryImpl.ts
index bf8b527..8e6c647 100644
--- a/src/SupportCodeLibraryImpl.ts
+++ b/src/SupportCodeLibraryImpl.ts
@@ -12,7 +12,7 @@ import type {
   MatchedStep,
   SupportCodeLibrary,
   UndefinedParameterType,
-} from './types'
+} from './types.js'
 
 type OrderedEnvelope = {
   order: number
diff --git a/src/UndefinedError.spec.ts b/src/UndefinedError.spec.ts
index cb683fe..5417444 100644
--- a/src/UndefinedError.spec.ts
+++ b/src/UndefinedError.spec.ts
@@ -1,8 +1,8 @@
 import type { PickleStep, Snippet } from '@cucumber/messages'
 import { expect } from 'chai'
 
-import type { UndefinedStep } from './types'
-import { UndefinedError } from './UndefinedError'
+import type { UndefinedStep } from './types.js'
+import { UndefinedError } from './UndefinedError.js'
 
 describe('UndefinedError', () => {
   it('should create an error message with the step text', () => {
diff --git a/src/UndefinedError.ts b/src/UndefinedError.ts
index 1ceb0c6..11c0b46 100644
--- a/src/UndefinedError.ts
+++ b/src/UndefinedError.ts
@@ -1,6 +1,6 @@
 import type { Snippet } from '@cucumber/messages'
 
-import type { UndefinedStep } from './types'
+import type { UndefinedStep } from './types.js'
 
 /**
  * Represents an error that occurs when no step definitions are found matching the text of a step
diff --git a/src/buildSupportCode.spec.ts b/src/buildSupportCode.spec.ts
index 796ddd9..77156d7 100644
--- a/src/buildSupportCode.spec.ts
+++ b/src/buildSupportCode.spec.ts
@@ -10,8 +10,8 @@ import {
 import { expect } from 'chai'
 import sinon from 'sinon'
 
-import { buildSupportCode } from './buildSupportCode'
-import type { SupportCodeLibrary } from './types'
+import { buildSupportCode } from './buildSupportCode.js'
+import type { SupportCodeLibrary } from './types.js'
 
 describe('buildSupportCode', () => {
   let newId: () => string
diff --git a/src/buildSupportCode.ts b/src/buildSupportCode.ts
index 40c447b..d23bae5 100644
--- a/src/buildSupportCode.ts
+++ b/src/buildSupportCode.ts
@@ -1,7 +1,7 @@
 import { IdGenerator } from '@cucumber/messages'
 
-import { SupportCodeBuilderImpl } from './SupportCodeBuilderImpl'
-import type { SupportCodeBuilder, SupportCodeOptions } from './types'
+import { SupportCodeBuilderImpl } from './SupportCodeBuilderImpl.js'
+import type { SupportCodeBuilder, SupportCodeOptions } from './types.js'
 
 /**
  * Start building up a library user-defined support code
diff --git a/src/index.ts b/src/index.ts
index fcf3c14..3d7c390 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,6 +1,6 @@
-export * from './AmbiguousError'
-export * from './buildSupportCode'
-export * from './DataTable'
-export * from './makeTestPlan'
-export * from './types'
-export * from './UndefinedError'
+export * from './AmbiguousError.js'
+export * from './buildSupportCode.js'
+export * from './DataTable.js'
+export * from './makeTestPlan.js'
+export * from './types.js'
+export * from './UndefinedError.js'
diff --git a/src/makeTestPlan.spec.ts b/src/makeTestPlan.spec.ts
index 8e5ba5a..91f8758 100644
--- a/src/makeTestPlan.spec.ts
+++ b/src/makeTestPlan.spec.ts
@@ -3,9 +3,9 @@ import { expect, use } from 'chai'
 import sinon from 'sinon'
 import sinonChai from 'sinon-chai'
 
-import { parseGherkin } from '../test/parseGherkin'
-import { buildSupportCode } from './buildSupportCode'
-import { makeTestPlan } from './makeTestPlan'
+import { parseGherkin } from '../test/parseGherkin.js'
+import { buildSupportCode } from './buildSupportCode.js'
+import { makeTestPlan } from './makeTestPlan.js'
 
 use(sinonChai)
 
diff --git a/src/makeTestPlan.ts b/src/makeTestPlan.ts
index a4c9b26..62633e9 100644
--- a/src/makeTestPlan.ts
+++ b/src/makeTestPlan.ts
@@ -22,7 +22,7 @@ import type {
   SupportCodeLibrary,
   TestPlanIngredients,
   TestPlanOptions,
-} from './types'
+} from './types.js'
 
 /**
  * Make an executable test plan for a Gherkin document
diff --git a/test/parseGherkin.ts b/test/parseGherkin.ts
index 14bcbc9..75c2012 100644
--- a/test/parseGherkin.ts
+++ b/test/parseGherkin.ts
@@ -7,7 +7,9 @@ export function parseGherkin(
   file: string,
   newId: () => string = IdGenerator.uuid()
 ): { gherkinDocument: GherkinDocument; pickles: ReadonlyArray<Pickle> } {
-  const data = fs.readFileSync(path.join(__dirname, '..', 'testdata', file), { encoding: 'utf-8' })
+  const data = fs.readFileSync(path.join(import.meta.dirname, '..', 'testdata', file), {
+    encoding: 'utf-8',
+  })
   const builder = new AstBuilder(newId)
   const matcher = new GherkinClassicTokenMatcher()
   const parser = new Parser(builder, matcher)