Chez Scheme is both a programming language and a high-performance implementation of that language. The language is a superset of R6RS Scheme with numerous extensions, including native threads, non-blocking I/O, local modules, and much more. Chez Scheme compiles source expressions incrementally to machine code, providing the speed of compiled code in an interactive system. The system is intended to be as reliable and efficient as possible, with reliability taking precedence over efficiency if necessary.