A study is carried out into the principal factors involved in the load and frequency control of interconnected power systems. Expressions are developed to give the magnitude of deviation of tie-line power flows and system frequency during times of disturbance in terms of power system and control system parameters. Further expressions are developed to indicate the quality of control likely to be achieved with the given set of parameters. The results of tests carried out on the load-frequency controls of the Saskatchewan-Manitoba-Northwestern Ontario power interconnection are included and are compared where possible to the theories presented and the expressions developed.