Experiments are described in which a plasma is used as an impedance element in an active Fabry‐Perot microwave resonator which contains a positive feedback loop. The arrangement functions as aQmultiplier or as an oscillator, with the plasma acting as a power or frequency‐determining element. The characteristics of the active Fabry‐Perot resonator are discussed in connection with diagnostic methods for pulsed and steady‐state plasmas.