As the use of the millimeter wave (MMW) bands increases, the need for better spectral control of sources also increases because of widening application of this spectrum to Doppler radar, communication, and measurement systems. As the millimeter bands become more crowded, better frequency control will become necessary to avoid interference. There is also a real advantage to be gained in source power stability by phase-locking; locked sources typically exhibit output power stabilities of a few tenths of one percent. Phase-locking of MMW sources to a frequency standard also provides the basis for frequency measurements which are perhaps the most accurate measurements of any physical quantity ever made, with accuracies of 1 X 10-12 being attainable. Phase-Lock circuits using both linear and digital techniques are available to the MMW system designer, and both types are discussed in this paper along with some typical results obtained.