EPG-sim is a newly-developed set of tools that performs execution-driven critical path simulation, trace generation, and simulation for serial, optimistically parallelized , anda parallel applications codes. Theses through the use of intelligent source-level instrumentation. The ability to perform execution-driven simulations driven by optimistically parllelized codes, the ability to execute these simuations on parallel hosts, the use of source-level instrumentation. and the integration of the cabilities provided by EPG-sim are among the novel contributions of theis work. EPG-sim has important uses in studying parallel architectures, parallelizing compilers, and parallel applications.