In this paper, a new approach to the design of nonlinear microwave oscillators is presented which allows optimum large-signal performance to be predicted. The procedure consists in determining in the power space a closed volume including all the allowed powers at terminal ports of the nonlinear device. The boundary of this volume, called the "characteristic surface", permits one to deduce the optimum powers and then the corresponding loads. The technique has been applied to oscillator circuits. Comparisons of Tunnel diode and Field Effect Transistor computed values with previously published results from the technical literature, as well as the agreement with experimental results, serve to validate the method.<<ETX>>