Since 2013, the PhET sims have been developed in HTML5, which is the language we recommend.
HTML5 Source Code
Or join in the discussion at our Developing Interactive Simulations in HTML5 Google Group:Developing Interactive Simulations in HTML5 Google Group
Getting started with HTML5 sim development video
This video is a quick guide to getting started developing with PhET on Windows. First, this video shows how to clone the PhET Libraries from GitHub. Next, it shows how to download and run a web server on your development machine. Finally, it shows how to launch an example simulation provided by PhET to test that everything is working nicely.Watch the video on youtube
Java/Flash/Flex Source Code (Legacy)
The PhET Java, Flash, and Flex source code lives in a Subversion repository. For instructions on how to browse the source code online or checkout the Subversion repository refer to our Legacy Java/Flash/Flex Source Code Document:Legacy Java/Flash/Flex Source Code Document