The validation of simulation models is not often treated to any great depth in reports on their use. This paper considers the problems involved in the validation and calibration of several classes of simulation models. Particular stress is laid on the checking of the internal consistency of the model against the problem as posed by the model designer. Distinctions are drawn between fitting, calibration, and validation of a model, and some of the implica tions for forecasting using such a model are discuss ed. Systematic errors may be introduced in any of these stages, and it is argued that the estimation of their magnitude is an intrinsic part of the validation process. Distinctions are drawn between validation procedures appropriate to discrete-event and continuous-simulation models, and to dynamic and cross-sectional structures in the systems modelled.