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It is shown that the theory of the maser can be simply presented in terms of the concept of the radiation reaction field. This is the radiation field produced in a cavity or wave guide by the presence of the sample's electromagnetic moment. Its reaction back on the sample causes coherence effects such as those called ``superradiance'' by Dicke. Two amplifiers using the radiation reaction field in different ways from the usual solid‐state maser are discussed. One of them takes advantage of the nonlinearity of the equations of motion to make a modulated coupling between two cavity modes by means of a sample with three energy levels such as is used in Bloembergen's maser. The resulting amplifying mixer behaves the same as (but is different in principle from) Suhl's ferromagnetic amplifier. A second type of amplifier employs the reaction field to destroy the circular polarization of a paramagnetic resonance. It is then possible to make an amplifier from this using the principle of the subharmonic generator.