The theory of monotonic scheduling is extended to hydrothermal systems with interconnections to adjoining power systems. The exposition is limited to the two most common types of interconnection arrangements, although the methods can be applied to other cases as well. All systems entering into this model are assumed to have thermal generation on line for the entire day. Variable-head hydraulic stations can be included in the model. Since energy may be bought and sold across interconnections, the criterion of an optimum schedule is maximum system profit rather than minimum fuel cost.