Simulation is one method used to derive control rules. It is non-optimal. One way of circumventing the non-optimality is to employ a “control band", which may in some respects be likened to a permissible deviation from the mean (the control rule). The concept is applicable to both on- and off-stream reservoirs. The systems where the idea was developed were the Ely Ouse-Essex and later Rutland Water in the UK. The start was a “family” of simulation-derived operating rules for each reservoir. Both main frame and microcomputers were used for the Rutland Water simulation and subsequent operations. The aim was to develop an operating strategy for deployment by the controllers. Only in severe climatic abnormality would the hydrologist need to modify the programme. Warnings that problems were developing would be highlighted when actual reservoir levels were seen to fall outside the “control band". The paper gives reasons for making use of simulation and indicates some of the benefits. Possible future work to upgrade both the operation and the development of both schemes is suggested. Although simulation was used in the two cases quoted, the basic idea may also be readily adapted to forms of linear programming. Continued development of the concept of the “control band” is possible.