The basic operating principles of Cˇerenkov maser oscillators are briefly reviewed and the experimental performance of a 3‐mm device is discussed. A power level of approximately 100 kW was achieved at 88 GHz and voltage tuning from 84 to 128 GHz on the fundamental TM01mode was observed. Operation on higher‐order modes at frequencies up to 300–320 GHz was demonstrated, and a two‐stage buncher‐amplifier configuration was investigated.