The article discuses certain selectad problems connected with methodology of designing computer simulatlon systems. There were shown one of concapts concerning documantation of stages in conatruction of a Computer simulation model in comparison with methodology of designing electronic data processing system at the level of en industrial enterprise. The author descussed the way of creating a conceptual model for the mass service system, network of operations of a model simulating behaviour of this system, and the problem of generating input data in conditions of appearance of randomness of phenomena. Finally. attention ia focussed on the way of using Computer simulation systems.