MOUSE (an acronym for Modular Oriented Uncertainty System) deals with the problem of uncertainties in models that consist of one or more algebraic equations. It was especially designed for use by those with little or no knowledge of computer languages, programming or simulation. It is designed to be run on almost any personal computer, easy and fast to learn, and has all of the features needed for substantive uncertainty analysis. Moreover, a series of unique companion utility programs write much of the necessary computer code for the user, help in analyzing sample data to determine the probability distributions that best fit that data, check each program for errors in syntax, and assist in finding logical errors in the model that is subject to uncertainty. Some typical examples of the use of MOUSE within the U.S. Environmental Protection Agency include: studying the migration of pollution plumes in streams, establishing regulations for hazardous wastes in landfills, and estimating pollution control costs.