Computer-based clinical simulations have been used in medical education for the past 25 years. During this period, the technology has evolved from mainframe computers to microcomputers to multimedia. All designers of simulations must decide which elements of reality to include explicitly in a simulated case, which to leave to the user's imagination, and when to intervene for educational purposes. Once these decisions are made, developers of simulations have many options for structuring the simulation itself. They can develop simulations with single or multiple patient encounters, with menu or natural-language requests for data, with varying levels of volunteered information about the simulated patient, with interpreted or uninterpreted clinical findings, with deterministic or probablistic evolution of the case, with various ways to give users feedback about their progress through the case, and with manual or automated creation of specific cases. Simulations derive their specific character from how these options are implemented.