The development of a simulation system that would provide both the interactive, user-oriented features of analog computers and the precision, memory, and computing power of the digital computer has been a design objective for many years. This paper reviews the background of present-day continuous systems simulation techniques, with emphasis on interactive, graphic-oriented systems. The state of the art is discussed, particularly as it concerns user convenience, interactive features, the use of block diagram descriptions, and graphic displays. BIOMOD, a system recently developed for simulation of biological systems, is used as an example. Limitations of present-day systems and prospects for near-future development are discussed.