This paper examines how multi-product, multi-level production control systems may be represented in terms of discrete linear control theory models. The transfer functions are derived for material requirements planning (MRP) and re-order cycle systems. The performance of each type of system is derived under selected conditions and the way in which a general system may be analysed is described. The method has proved easy to use. Computer programs exist which enable one to assemble the elements of proposed forecasting, inventory and production control systems, and hence determine the stability and transient and steady state responses to standard inputs. This has proved to be a quick and efficient way of evaluating alternative production systems and examining whether better methods of, for example, stock re-ordering or forecasting are required.