See the project page online for more information: http://code.google.com/p/jbox2d/ If you've downloaded this an archive, you should find the built java jars in the 'target' directories of each project. jbox2d-library - this is the main physics library. The only dependency is the SLF4J logging library. jbox2d-serialization - this adds serialization tools. Requires google's protocol buffer library installed to fully build (http://code.google.com/p/protobuf/). jbox2d-testbed - A simple framework for creating and running physics tests.