Development

GitHub: MuseScore's source code and development hub (commits feed / RSS). The preferred way to contribute is via pull request (currently open PRs). Read our Git Workflow page to learn how to make a pull request. You need to sign the MuseScore CLA for your request to be merged.
Developers' handbook: All information regarding MuseScore development (including build/compile instructions for supported OSes). If you have a musescore.org account, you can change these pages in a wiki style. Be bold and add your knowledge.

Plugins

Plugin development: Reference information for writing MuseScore plugins (for 1.3 and older only).
Most documentation for 2.0+ is built-in to the Plugin Creator. Here is a quick guide: Plugins for 2.x.
Plugins for 3.0: Stub for changes to the plugin framework for MuseScore 3.0.

Get help from other developers

To make your first contribution, or to talk about something you want to do to MuseScore, you can contact the other developers.

Development and Technology Preview forum: Get in touch with all the MuseScore developers
Developer group chats using Telegram

Testing

Issue tracker: Post and track MuseScore bug reports, feature requests or tasks. If you are interested in helping test new features and for bugs, see Comparison of stable, beta, and development versions. This helps improve the quality of the next stable release.

Documentation

The MuseScore community maintains a collaborative written handbook. Any logged in user can edit and improve the handbook pages. If you see a mistake or something missing in the handbook, don't hesitate to fix it or extend it—click the ⠇ to the right of the title, choose "Edit," and improve the page. In cases where you would like to discuss changes, post in the documentation forum.

Translating

People all over the world use MuseScore, thanks to a vibrant community of translators. If you want to help with an existing translation, or start a new one, see the instructions below. Please ask any questions on the forums, or developers mailing list and share initial work - particularly to avoid further workload, should changes be required.

Handbook and website

If you are interested in translating the website, or the software manual, then introduce yourself on the translation forum. We will then give you access to edit and create new pages for your language. Also see the extensive Translation instructions.

Software

The MuseScore community uses a translation server, making it simpler to collaboratively localize MuseScore. If you want to start translating to a language that is not yet listed, leave a request on the Translation forum so the language can be added.
There's also a MuseScore translators' group chat using Telegram