The main purposes of modelling are a reasonable understanding of the physical processes involved and a careful evaluation of reasonable alternative designs. The central concern is credibility.Inreasingly, government and municipal engineers involved in urban drainage design supervise studies in which sophisticated computer models are being used by specialists. Because of the large variety of programs available for modelling, the rapid evolution of the models, and because of their complex structures, the studies are becoming increasingly difficult to manage.In the initial study terms of reference, the following component study activities could be specified in detail when bids are requested: problem review, study objectives, performance criteria, requisite accuracy, review of available programs, available data and study resources, program selection criteria, model verification, model calibration, model validation, minimum level of discretization, sensitivity analysis, data preparation and output interpretation, documentation of the modifed program actually used, and preparation of machine readable input and output files for archiving. Many of these activities will be carried out by a reputable engineer in any case.By including these terms of reference, all consultants bidding will be assured that their costs will be met, and clients will be insuring their investment in design engineering with a modest premium. Each of these points is discussed using examples appropriate to stormwater management modelling studies. However, a number of the concepts can be applied generally to other disciplines using computer-based modelling.