A method for simulating systems that have components controlled in an ON/OFF manner is presented. The method is particularly suited to systems with an identifiable component structure such as those found in network problems. The primary feature of the method is that control strategies are modelled separately from the dynamics of the physical system so that various strategies may be added, modified or deleted without affecting either the model of the physical system or the computer code. Control strategies are expressed as combinations of comparators, logic operators (gates) and self-defined control functions such as hysteresis. The complete mathematical model for representing a control strategy, the procedure for automatically formulating the resulting system of control equations, directly from the control strategy, and the interconnection of the physical system model and the control model, are presented. Pipe-network examples are given.