GoldSim is a powerful and flexible Windows-based computer program for carrying out probabilistic simulations of complex systems to support management and decisionmaking in engineering, science and business. The program is highly graphical, highly extensible, able to directly represent uncertainty, and allows you to create compelling presentations of your model. Although GoldSim can be used to solve a wide variety of complex problems, it is particularly well-suited (and was originally developed) to support evaluation of complex environmental systems. Powerful contaminant transport features allow nearly any kind of natural or man-made environmental system to be simulated. This paper provides a brief overview of GoldSim, with special emphasis on environmental applications.