A discussion of the basic concepts of the techniques of digital computer simulation: the study provides a rationale for simulation, discusses the design and construction of simulation models, and relates simulation as a technique to current problems in simulation technology. Simulation is defined as the use of a numerical model to study the behavior of a system as it operates over time. Emphasis is on models that are implemented on digital computers --that is, models that operate by advancing a system through time in discrete steps rather than continuously, as is done with analog computers. Because it is an experimental numerical technique, simulation is usually more expensive than analytic solutions; nevertheless, it is widely used for complex problems that cannot be solved analytically, such as those encountered in engineering and management studies. Even certain structurally simple analytical models must be simulated because their statistical properties do not admit analytic solution. Simple models are broad in scope and contain few assumptions; complex, highly structured models are narrow in scope and contain many assumptions. Before a simulation model is designed, it is important to determine what use will be made of it and what are the requirements of accuracy and precision. A model should only be as detailed as is necessary to answer the questions at hand.