The basis for analysis of all chemical and metallurgical processes is the mass and energy balance. Plant design, capital costs, and technical evaluations are all dependent on such calculations. METSIM is a general-purpose process simulation system designed to assist the engineer in performing mass and energy balances of complex processes. METSIM uses an assortment of computational methods to effect an optimum combination of complexity, user time, and computer resources usage.

METSIM originated as a metallurgical process simulation program, written to perform mass balances around the major unit operations of complex process flowsheets. Application of the program proved so successful that it was expanded to include detailed heat balances, chemistry, process controls, equipment sizing, cost estimation, and process analysis. The unique nature of the programming language, APL, allows modification and expansion of the system with minimum effort and permits the incorporation of continuing technological innovations in process simulation.

Many diverse processes have been modeled with METSIM, including: