Two electronic circuits for frequency stabilization of electronically tunable microwave oscillators are described and discussed. One of these uses a microwave circuit equivalent to the lowfrequency discriminator, in conjunction with a d.c. amplifier, to control the oscillator frequency at the frequency of a cavity resonator. The other circuit obtains frequency control of the oscillator by the cavity through a circuit operating almost entirely at an intermediate frequency. With both systems, frequency modulation of a highly degenerative type is provided. The resulting stability over long periods is essentially that of the cavity. The stability over short periods is such that the signal obtained occupies a band of less than 1 part in 108in width. The practical limit to the stabilization obtainable with given components is estimated, and several applications are suggested.