Currently many interconnection networks and parallel algorithms exist for message passing computers. Users of these machines wish to determine which message passing computer is best for a given job, and how it will scale with the number of processors and algorithm size. This paper describes a general purpose simulator for message passing multiprocessors (Parsim), which facilitates system modelling. A structured method for simulator design has been used which gives Parsim the ability to easily simulate different topologies and algorithm combinations. This is illustrated by applying Parsim to a number of algorithms on a variety of topologies. Parsim is then used to predict the performance of the new IBM SP2 parallel computer, with topologies ranging up to 1024 processors.