We describe the implementation of “MARS,” a GUI, Java-based simulator for the MIPS assembly language. MIPS, the computer architecture underlying the simulated assembly language, is widely used in industry and is the basis of the popular textbook Computer Organization and Design [6], used at over 400 universities. The MARS simulator has been implemented with characteristics that are especially useful to undergraduate computer science students and their instructors. Categories and Subject Descriptors C.0 [Computer Systems Organization]: General – Hardware/software interfaces, Instruction set design (RISC), Modeling of computer architecture. General Terms Languages Keywords Architecture, assembly language, simulation, MIPS