This book describes the structure of simulators suitable for use in the design of digital electronic systems. It includes the compiled code and event driven algorithms for digital electronic system simulators, together with timing verification. Limitations of the structures are also discussed. An introduction to the problems of designing models is included, partly to point to how user models might be constructed for application specific integrated circuits (ASICs) and so on, and partly to expose the limitations of the modelling process. As a guide to the use of simulators the book includes chapters which introduce the subjects of testing and design for testability. A major chapter is devoted to fault simulation. The text has an introduction to hardware accelerators and modellers.