diff --git a/.travis.yml b/.travis.yml
index b18bae5..21a7e31 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,4 +1,8 @@
 sudo: false
+before_install:
+  - npm install full-icu
+before_script:
+  - export NODE_ICU_DATA=node_modules/full-icu
 language: node_js
 node_js:
   - '6'
diff --git a/index.js b/index.js
index 9278dbe..2788730 100644
--- a/index.js
+++ b/index.js
@@ -1,11 +1,13 @@
 'use strict';
 const UNITS = ['B', 'kB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
 
-module.exports = num => {
+module.exports = (num, options) => {
 	if (!Number.isFinite(num)) {
 		throw new TypeError(`Expected a finite number, got ${typeof num}: ${num}`);
 	}
 
+	options = Object.assign({}, options);
+
 	const neg = num < 0;
 
 	if (neg) {
@@ -13,12 +15,32 @@ module.exports = num => {
 	}
 
 	if (num < 1) {
-		return (neg ? '-' : '') + num + ' B';
+		const numStr = toLocaleString(num, options.locale);
+		return (neg ? '-' : '') + numStr + ' B';
 	}
 
 	const exponent = Math.min(Math.floor(Math.log10(num) / 3), UNITS.length - 1);
-	const numStr = Number((num / Math.pow(1000, exponent)).toPrecision(3));
+	num = Number((num / Math.pow(1000, exponent)).toPrecision(3));
+	const numStr = toLocaleString(num, options.locale);
+
 	const unit = UNITS[exponent];
 
 	return (neg ? '-' : '') + numStr + ' ' + unit;
 };
+
+/**
+ * Formats the given number using number.toLocaleString(..).
+ * If locale is a string, the value is expected to be a locale-key (e.g. 'de').
+ * If locale is true, the system default locale is used for translation.
+ * If no value for locale is specified, the number is returned unmodified.
+ */
+function toLocaleString(num, locale) {
+	let result = num;
+	if (typeof locale === 'string') {
+		result = num.toLocaleString(locale);
+	} else if (locale === true) {
+		result = num.toLocaleString();
+	}
+
+	return result;
+}
diff --git a/readme.md b/readme.md
index fd7b525..490bd29 100644
--- a/readme.md
+++ b/readme.md
@@ -25,8 +25,28 @@ prettyBytes(1337);
 
 prettyBytes(100);
 //=> '100 B'
+
+// Localize output using german locale
+prettyBytes(1337, { locale: 'de' });
+//=> '1,34 kB'
 ```
 
+## API
+### prettyBytes(input, [options])
+#### input
+Type: `number`
+
+The number to format.
+
+#### options
+##### locale
+Type: `boolean || string`
+Default: `false` / no localization
+
+- `string`: Expects a [BCP 47 language tag](https://en.wikipedia.org/wiki/IETF_language_tag) (e.g. `en`, `de`, ...)
+- `boolean`: If `true`: Localize the output using the system/browser locale.
+
+**Note:** Localization should generally work in browsers. Node.js needs to be [built](https://github.com/nodejs/node/wiki/Intl) with `full-icu` or `system-icu`. Alternatively, the [full-icu](https://github.com/unicode-org/full-icu-npm) module can be used to provide support at runtime.
 
 ## Related
 
diff --git a/test.js b/test.js
index c3d7f03..99f2dc9 100644
--- a/test.js
+++ b/test.js
@@ -31,3 +31,35 @@ test('supports negative number', t => {
 	t.is(m(-999), '-999 B');
 	t.is(m(-1001), '-1 kB');
 });
+
+test('localized output', t => {
+	t.is(m(-0.4, {locale: 'de'}), '-0,4 B');
+	t.is(m(0.4, {locale: 'de'}), '0,4 B');
+	t.is(m(1001, {locale: 'de'}), '1 kB');
+	t.is(m(10.1, {locale: 'de'}), '10,1 B');
+	t.is(m(1e30, {locale: 'de'}), '1.000.000 YB');
+
+	t.is(m(-0.4, {locale: 'en'}), '-0.4 B');
+	t.is(m(0.4, {locale: 'en'}), '0.4 B');
+	t.is(m(1001, {locale: 'en'}), '1 kB');
+	t.is(m(10.1, {locale: 'en'}), '10.1 B');
+	t.is(m(1e30, {locale: 'en'}), '1,000,000 YB');
+
+	t.is(m(-0.4, {locale: true}), '-0.4 B');
+	t.is(m(0.4, {locale: true}), '0.4 B');
+	t.is(m(1001, {locale: true}), '1 kB');
+	t.is(m(10.1, {locale: true}), '10.1 B');
+	t.is(m(1e30, {locale: true}), '1,000,000 YB');
+
+	t.is(m(-0.4, {locale: false}), '-0.4 B');
+	t.is(m(0.4, {locale: false}), '0.4 B');
+	t.is(m(1001, {locale: false}), '1 kB');
+	t.is(m(10.1, {locale: false}), '10.1 B');
+	t.is(m(1e30, {locale: false}), '1000000 YB');
+
+	t.is(m(-0.4, {locale: undefined}), '-0.4 B');
+	t.is(m(0.4, {locale: undefined}), '0.4 B');
+	t.is(m(1001, {locale: undefined}), '1 kB');
+	t.is(m(10.1, {locale: undefined}), '10.1 B');
+	t.is(m(1e30, {locale: undefined}), '1000000 YB');
+});