The source code of Picard is maintained in Git and you can get it with all its history using:
git clone git://github.com/musicbrainz/picard.git
or you can fork or clone the repository from GitHub in the normal GitHub way. Pull requests and discussion on GitHub is generally preferred and all such requests will be considered.
If you are interested in providing development assistance, contact us in the MetaBrainz chat. To help by reporting bugs or finding a list of available tickets to work on, please have a look at the Probleemoplossing page.
If you want to install Picard from source on Windows, there is Building Picard for Windows.
For macOS there is Building Picard on macOS.
Picard is licensed under the GPL 2.0 or later.