MOUSE (Modular Oriented Uncertainty SystEm) is a Monte Carlo based simulation system designed to deal with the problem of uncertainties in Environmental engineering calculations, such as a set of engineering cost or risk analysis equations. It was especially intended for use by individuals with little or no knowledge on computer languages, programming, or simulation. However, it is also of benefit to those experienced in simulation and simulation languages who desire a quick and easy method to produce standard Monte Carlo simulation output. MOUSE system utilities run on MS-DOS-based personal computers, but MOUSE programs themselves can be run on any computer equipped with a FORTRAN compiler. It is easy and fast to learn and has all of the features needed for substantive uncertainty analysis, such as built-in probability distributions, plotting and graphing capabilities, sensitivity analysis, and interest functions for cost analyses.