diff --git a/index.d.ts b/index.d.ts
new file mode 100644
index 0000000..d1e1aaf
--- /dev/null
+++ b/index.d.ts
@@ -0,0 +1,55 @@
+declare namespace findUp {
+	interface Options {
+		/**
+		Directory to start from.
+
+		@default process.cwd()
+		*/
+		readonly cwd?: string;
+	}
+}
+
+declare const findUp: {
+	/**
+	Find a file or directory by walking up parent directories.
+
+	@param filename - Filename of the file or an array of files to find.
+	@returns Either the first filepath found (by respecting the order of `filename`s) or `null`.
+
+	@example
+	```
+	// /
+	// └── Users
+	// 		└── sindresorhus
+	// 				├── unicorn.png
+	// 				└── foo
+	// 						└── bar
+	// 								├── baz
+	// 								└── example.js
+
+	// example.js
+	import findUp = require('find-up');
+
+	(async () => {
+		console.log(await findUp('unicorn.png'));
+		//=> '/Users/sindresorhus/unicorn.png'
+
+		console.log(await findUp(['rainbow.png', 'unicorn.png']));
+		//=> '/Users/sindresorhus/unicorn.png'
+	})();
+	```
+	*/
+	(filename: string | string[], options?: findUp.Options): Promise<
+		string | null
+	>;
+
+	/**
+	Synchronously find a file or directory by walking up parent directories.
+
+	@param filename - Filename of the file or an array of files to find.
+	@returns Either the first filepath found (by respecting the order of `filename`s) or `null`.
+	*/
+	sync(filename: string | string[], options?: findUp.Options): string | null;
+};
+
+export = findUp;
diff --git a/index.js b/index.js
index 8e83819..de62b65 100644
--- a/index.js
+++ b/index.js
@@ -2,45 +2,45 @@
 const path = require('path');
 const locatePath = require('locate-path');
 
-module.exports = (filename, opts = {}) => {
-	const startDir = path.resolve(opts.cwd || '');
-	const {root} = path.parse(startDir);
+module.exports = (filename, options = {}) => {
+	const startDirectory = path.resolve(options.cwd || '');
+	const {root} = path.parse(startDirectory);
 
 	const filenames = [].concat(filename);
 
 	return new Promise(resolve => {
-		(function find(dir) {
-			locatePath(filenames, {cwd: dir}).then(file => {
+		(function find(directory) {
+			locatePath(filenames, {cwd: directory}).then(file => {
 				if (file) {
-					resolve(path.join(dir, file));
-				} else if (dir === root) {
+					resolve(path.join(directory, file));
+				} else if (directory === root) {
 					resolve(null);
 				} else {
-					find(path.dirname(dir));
+					find(path.dirname(directory));
 				}
 			});
-		})(startDir);
+		})(startDirectory);
 	});
 };
 
-module.exports.sync = (filename, opts = {}) => {
-	let dir = path.resolve(opts.cwd || '');
-	const {root} = path.parse(dir);
+module.exports.sync = (filename, options = {}) => {
+	let directory = path.resolve(options.cwd || '');
+	const {root} = path.parse(directory);
 
 	const filenames = [].concat(filename);
 
 	// eslint-disable-next-line no-constant-condition
 	while (true) {
-		const file = locatePath.sync(filenames, {cwd: dir});
+		const file = locatePath.sync(filenames, {cwd: directory});
 
 		if (file) {
-			return path.join(dir, file);
+			return path.join(directory, file);
 		}
 
-		if (dir === root) {
+		if (directory === root) {
 			return null;
 		}
 
-		dir = path.dirname(dir);
+		directory = path.dirname(directory);
 	}
 };
diff --git a/index.test-d.ts b/index.test-d.ts
new file mode 100644
index 0000000..6ae68d1
--- /dev/null
+++ b/index.test-d.ts
@@ -0,0 +1,16 @@
+import {expectType} from 'tsd';
+import findUp = require('.');
+
+expectType<Promise<string | null>>(findUp('unicorn.png'));
+expectType<Promise<string | null>>(findUp('unicorn.png', {cwd: ''}));
+expectType<Promise<string | null>>(findUp(['rainbow.png', 'unicorn.png']));
+expectType<Promise<string | null>>(
+	findUp(['rainbow.png', 'unicorn.png'], {cwd: ''})
+);
+
+expectType<string | null>(findUp.sync('unicorn.png'));
+expectType<string | null>(findUp.sync('unicorn.png', {cwd: ''}));
+expectType<string | null>(findUp.sync(['rainbow.png', 'unicorn.png']));
+expectType<string | null>(
+	findUp.sync(['rainbow.png', 'unicorn.png'], {cwd: ''})
+);
diff --git a/package.json b/package.json
index a708f57..6d54402 100644
--- a/package.json
+++ b/package.json
@@ -13,10 +13,11 @@
 		"node": ">=6"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"find",
@@ -43,8 +44,9 @@
 		"locate-path": "^3.0.0"
 	},
 	"devDependencies": {
-		"ava": "*",
+		"ava": "^1.4.1",
 		"tempy": "^0.2.1",
-		"xo": "*"
+		"tsd": "^0.7.2",
+		"xo": "^0.24.0"
 	}
 }
diff --git a/test.js b/test.js
index e6971aa..6b1e224 100644
--- a/test.js
+++ b/test.js
@@ -2,32 +2,35 @@ import fs from 'fs';
 import path from 'path';
 import test from 'ava';
 import tempy from 'tempy';
-import m from '.';
+import findUp from '.';
 
 const name = {
-	pkgDir: 'find-up',
-	pkg: 'package.json',
-	fixtureDir: 'fixture',
+	packageDirectory: 'find-up',
+	packageJson: 'package.json',
+	fixtureDirectory: 'fixture',
 	baz: 'baz.js',
 	qux: 'qux.js'
 };
 
 // These paths are relative to the project root
-const rel = {
-	fixtureDir: name.fixtureDir
+const relative = {
+	fixtureDirectory: name.fixtureDirectory
 };
-rel.baz = path.join(rel.fixtureDir, name.baz);
-rel.qux = path.join(rel.fixtureDir, name.qux);
-rel.barDir = path.join(rel.fixtureDir, 'foo', 'bar');
+relative.baz = path.join(relative.fixtureDirectory, name.baz);
+relative.qux = path.join(relative.fixtureDirectory, name.qux);
+relative.barDir = path.join(relative.fixtureDirectory, 'foo', 'bar');
 
-const abs = {
-	pkgDir: __dirname
+const absolute = {
+	packageDirectory: __dirname
 };
-abs.pkg = path.join(abs.pkgDir, name.pkg);
-abs.fixtureDir = path.join(abs.pkgDir, name.fixtureDir);
-abs.baz = path.join(abs.fixtureDir, name.baz);
-abs.qux = path.join(abs.fixtureDir, name.qux);
-abs.barDir = path.join(abs.fixtureDir, 'foo', 'bar');
+absolute.packageJson = path.join(absolute.packageDirectory, name.packageJson);
+absolute.fixtureDirectory = path.join(
+	absolute.packageDirectory,
+	name.fixtureDirectory
+);
+absolute.baz = path.join(absolute.fixtureDirectory, name.baz);
+absolute.qux = path.join(absolute.fixtureDirectory, name.qux);
+absolute.barDir = path.join(absolute.fixtureDirectory, 'foo', 'bar');
 
 // Create a disjoint directory, used for the not-found tests
 test.beforeEach(t => {
@@ -40,189 +43,189 @@ test.afterEach(t => {
 });
 
 test('async (child file)', async t => {
-	const filePath = await m(name.pkg);
+	const filePath = await findUp(name.packageJson);
 
-	t.is(filePath, abs.pkg);
+	t.is(filePath, absolute.packageJson);
 });
 
 test('sync (child file)', t => {
-	const filePath = m.sync(name.pkg);
+	const filePath = findUp.sync(name.packageJson);
 
-	t.is(filePath, abs.pkg);
+	t.is(filePath, absolute.packageJson);
 });
 
 test('async (child dir)', async t => {
-	const filePath = await m(name.fixtureDir);
+	const filePath = await findUp(name.fixtureDirectory);
 
-	t.is(filePath, abs.fixtureDir);
+	t.is(filePath, absolute.fixtureDirectory);
 });
 
 test('sync (child dir)', t => {
-	const filePath = m.sync(name.fixtureDir);
+	const filePath = findUp.sync(name.fixtureDirectory);
 
-	t.is(filePath, abs.fixtureDir);
+	t.is(filePath, absolute.fixtureDirectory);
 });
 
 test('async (child file, custom cwd)', async t => {
-	const filePath = await m(name.baz, {
-		cwd: rel.fixtureDir
+	const filePath = await findUp(name.baz, {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('sync (child file, custom cwd)', t => {
-	const filePath = m.sync(name.baz, {
-		cwd: rel.fixtureDir
+	const filePath = findUp.sync(name.baz, {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('async (child file, array, custom cwd)', async t => {
-	const filePath = await m([name.baz], {
-		cwd: rel.fixtureDir
+	const filePath = await findUp([name.baz], {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('sync (child file, array, custom cwd)', t => {
-	const filePath = m.sync([name.baz], {
-		cwd: rel.fixtureDir
+	const filePath = findUp.sync([name.baz], {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('async (first child file, array, custom cwd)', async t => {
-	const filePath = await m([name.qux, name.baz], {
-		cwd: rel.fixtureDir
+	const filePath = await findUp([name.qux, name.baz], {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.qux);
+	t.is(filePath, absolute.qux);
 });
 
 test('sync (first child file, array, custom cwd)', t => {
-	const filePath = m.sync([name.qux, name.baz], {
-		cwd: rel.fixtureDir
+	const filePath = findUp.sync([name.qux, name.baz], {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.qux);
+	t.is(filePath, absolute.qux);
 });
 
 test('async (second child file, array, custom cwd)', async t => {
-	const filePath = await m(['fake', name.baz], {
-		cwd: rel.fixtureDir
+	const filePath = await findUp(['fake', name.baz], {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('sync (second child file, array, custom cwd)', t => {
-	const filePath = m.sync(['fake', name.baz], {
-		cwd: rel.fixtureDir
+	const filePath = findUp.sync(['fake', name.baz], {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('async (cwd)', async t => {
-	const filePath = await m(name.pkgDir, {
-		cwd: abs.pkgDir
+	const filePath = await findUp(name.packageDirectory, {
+		cwd: absolute.packageDirectory
 	});
 
-	t.is(filePath, abs.pkgDir);
+	t.is(filePath, absolute.packageDirectory);
 });
 
 test('sync (cwd)', t => {
-	const filePath = m.sync(name.pkgDir, {
-		cwd: abs.pkgDir
+	const filePath = findUp.sync(name.packageDirectory, {
+		cwd: absolute.packageDirectory
 	});
 
-	t.is(filePath, abs.pkgDir);
+	t.is(filePath, absolute.packageDirectory);
 });
 
 test('async (cousin file, custom cwd)', async t => {
-	const filePath = await m(name.baz, {
-		cwd: rel.barDir
+	const filePath = await findUp(name.baz, {
+		cwd: relative.barDir
 	});
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('sync (cousin file, custom cwd)', t => {
-	const filePath = m.sync(name.baz, {
-		cwd: rel.barDir
+	const filePath = findUp.sync(name.baz, {
+		cwd: relative.barDir
 	});
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('async (nested descendant file)', async t => {
-	const filePath = await m(rel.baz);
+	const filePath = await findUp(relative.baz);
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('sync (nested descendant file)', t => {
-	const filePath = m.sync(rel.baz);
+	const filePath = findUp.sync(relative.baz);
 
-	t.is(filePath, abs.baz);
+	t.is(filePath, absolute.baz);
 });
 
 test('async (nested descendant dir)', async t => {
-	const filePath = await m(rel.barDir);
+	const filePath = await findUp(relative.barDir);
 
-	t.is(filePath, abs.barDir);
+	t.is(filePath, absolute.barDir);
 });
 
 test('sync (nested descendant dir)', t => {
-	const filePath = m.sync(rel.barDir);
+	const filePath = findUp.sync(relative.barDir);
 
-	t.is(filePath, abs.barDir);
+	t.is(filePath, absolute.barDir);
 });
 
 test('async (nested cousin dir, custom cwd)', async t => {
-	const filePath = await m(rel.barDir, {
-		cwd: rel.fixtureDir
+	const filePath = await findUp(relative.barDir, {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.barDir);
+	t.is(filePath, absolute.barDir);
 });
 
 test('sync (nested cousin dir, custom cwd)', t => {
-	const filePath = m.sync(rel.barDir, {
-		cwd: rel.fixtureDir
+	const filePath = findUp.sync(relative.barDir, {
+		cwd: relative.fixtureDirectory
 	});
 
-	t.is(filePath, abs.barDir);
+	t.is(filePath, absolute.barDir);
 });
 
 test('async (ancestor dir, custom cwd)', async t => {
-	const filePath = await m(name.fixtureDir, {
-		cwd: rel.barDir
+	const filePath = await findUp(name.fixtureDirectory, {
+		cwd: relative.barDir
 	});
 
-	t.is(filePath, abs.fixtureDir);
+	t.is(filePath, absolute.fixtureDirectory);
 });
 
 test('sync (ancestor dir, custom cwd)', t => {
-	const filePath = m.sync(name.fixtureDir, {
-		cwd: rel.barDir
+	const filePath = findUp.sync(name.fixtureDirectory, {
+		cwd: relative.barDir
 	});
 
-	t.is(filePath, abs.fixtureDir);
+	t.is(filePath, absolute.fixtureDirectory);
 });
 
 test('async (not found)', async t => {
-	const filePath = await m('somenonexistentfile.js');
+	const filePath = await findUp('somenonexistentfile.js');
 
 	t.is(filePath, null);
 });
 
 test('sync (not found)', t => {
-	const filePath = m.sync('somenonexistentfile.js');
+	const filePath = findUp.sync('somenonexistentfile.js');
 
 	t.is(filePath, null);
 });
@@ -230,7 +233,7 @@ test('sync (not found)', t => {
 // Both tests start in a disjoint directory. `package.json` should not be found
 // and `null` should be returned.
 test('async (not found, custom cwd)', async t => {
-	const filePath = await m(name.pkg, {
+	const filePath = await findUp(name.packageJson, {
 		cwd: t.context.disjoint
 	});
 
@@ -238,7 +241,7 @@ test('async (not found, custom cwd)', async t => {
 });
 
 test('sync (not found, custom cwd)', t => {
-	const filePath = m.sync(name.pkg, {
+	const filePath = findUp.sync(name.packageJson, {
 		cwd: t.context.disjoint
 	});