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The magnetron, invented in 1939, created the need for a microwave tube magnet which evolved from the early horn magnet to the U magnet, the E magnet, and finally the bowl magnet. A more recent innovation, the self-shielded cylindrical magnet, is now used on many crossed-field amplifiers. The klystron and the traveling wave tube created the need for permanent magnets with gaps longer than their diameters. The field straightener and the periodic permanent magnet facilitated the use of permanent magnets in focusing these linear beam tubes. The operation of magnetrons, crossed field amplifiers, klystrons, traveling wave tubes, and microwave ferrite devices, and the permanent magnets which are used in conjunction with these devices are described. Magnetic circuit design criteria, design methods, and the magnetization and stabilization of permanent magnets are discussed. The requirements of microwave devices have provided much of the incentive for the great advances in permanent magnet materials made in the last decade. The recent development of rare-earth-cobalt powders which exhibit the highest magnetocrystalline anisotropy ever recorded are of great interest to the microwave device designer.