The theory of reflex-klystron oscillators is given in detail. It includes a discussion of relations in. a loaded oscillator. It is shown that maximum efficiency for small amplitudes is given by η = 0.169M2i1/GcVo, where M is the coefficient of modulation of the gap, i1is the effective current, Gcis the shunt conductance of the unloaded resonator, and Vois the beam voltage. Possibilities of increasing efficiency are considered, including effects of grid transmission on effective current and on space charge, and effects of multiple electron transits.