The multilevel control of large systems intended to distribute between several control levels, the calculation of optimum set points and the use and calculation of optimum control laws are based upon the concept of decomposition-co-ordination of the overall optimisation problem. For the nonseparable problem examined, it is necessary to use a fomulation of the decomposition-co-ordination principle different from that used for separable problems. Decomposition methods (recognition of coupling variables, introduction of pseudovariables) and coordination algorithms are described briefly. The problem of optimum power scheduling in an hydroelectric system, representing an example of a typical nonseparable optimisation problem, is discussed. Various decomposition co-ordination methods are proposed, and a concrete example is examined which shows the effectiveness of multilevel control techniques applied to very highly nonseparable problems.