The Multiple Event Stream Simulator (MESS) is a software package written in Common Lisp, for general discrete event simulation. MESS is designed to be a substrate for experiments in real-time planning, where "real-time" means a simulation in which the thinking of one or more agents occurs concurrently with the ongoing simulation, and therefore the duration of their thinking must be modeled. It would perhaps be more accurate to call it "time-pressured planning." MESS itself is domain-independent, providing only the means to execute events, which move the simulation from state to state.