IMPATT diodes are presented as an alternate rf source for EPR. Their noise‐performance is shown to be comparable to klystrons while having the advantages of low cost, long life, and simple power supply requirements. The stability of the IMPATT diode is such that frequency locking is not usually necessary. Where needed, locking may be done with existing klystron lock boxes through the addition of a varactor. An IMPATT marginal oscillator with a sensitivity of about 2 × 1012spins/G is also described. The oscillator's sensitivity is essentially independent of power and it can be used with an external detector or in the self‐detection mode.