A basically tutorial point of view is taken in this general discussion. The author examines the basic concepts and principles of simulation and modelling and the application of digital computers to these tasks. Examples of existing simulations, a discussion of the applicability and feasibility of simulation studies, a review of simulation techniques, lists of the advantages and dangers in simulating and a classification of digital computer simulations are also presented. While the emphasis is on the use of digital computers, many of the principles examined hold for simulation done on analogue computers. No attempt is made, however, to compare the uses of the two types of machines for simulation studies. (Author/JN)