This paper reviews the state of the art of gallium-arsenide, impact avalanche transit-time (IMPATT) diodes which are used as oscillators or amplifiers. An outline of the history of these devices is first noted. The basic theoretical device designs are reviewed, and the various device-fabrication techniques discussed in detail. Some of the new results are reviewed, including some highlights of our new Read-diode work submitted for publication elsewhere. Additional comments are made concerning the reliability of GaAs IMPATT diodes. Finally, conclusions are offered, and projections made whic indicate that higher power with higher efficiencies may be available in the near future.