Synchronous generators operating at constant speed are used to supply the electric power systems of most modern aircraft. The constant speed salient pole generator inherently develops the output power in the desired form, a sine wave of 400 hertz frequency. The hydromechanical constant speed drive, a straightforward, rugged combination of a planetary gear set and hydraulic units, is most frequently chosen to achieve the constant speed. The design of the constant speed generating system has evolved through experience, development, and state-of-the-art advances. The current system, the integrated drive generator, has the advantages of weight, performance, reliability, and ease of maintenance. Continued engineering effort is dedicated to achieving further advances.