Many books deal with the techniques of designing, building and testing computer models and simulations, but few have seriously examined what models are based on, their inherent limitations, and their essential role in extending human knowledge. This book fills this need. It focuses on computer models throughout, yet its exposition of the nature and limits of modeling is entirely general. A chapter on the development of celestial mechanics models illustrates how models progress and are essential to natural science. Chapters on models of global climate, population, economics and warfare, illustrate both the possibilities and limits of modeling nature and similar deterministic processes in human affairs. The book also discusses the serious limitations placed on the simulation of human societies and their histories, since models must deal with both cultural and natural forces. The concluding chapter explores virtual realities as systems of interactive images generated by computer models.