JastAdd is an open source Java-based compiler compiler system. It is designed to support high-level extensible implementation of compilers and related tools like analyzers, transformation tools, etc. The JastAdd Extensible Java Compiler provides both a proof-of-concept that JastAdd can be used for large substantial projects, lika a complete Java compiler. It also illustrates extensibility and modularity in showing how different tools and language extensions can be implemented as extensions to other components..