A millimeter wave beam system, based on an 83 GHz gyrotron with superconducting magnets, produces up to 15 kW of CW power which can be focused to a spot size of several millimeters. This system has been used for a wide range of material processing experiments. One of these is brazing of poled piezoelectric ceramics to each other without significant heating and depoling of the ceramic components. Other experiments include millimeter wave joining of ceramics, and coating of metals and polymers. In the former case, joints of both similar and dissimilar materials have been demonstrated, both without and with additional materials (reactive brazes) in the joint. In the latter case, the short wavelength and absorption depth of the millimeter wave beam permit effective deposition of ceramic coatings on materials of lower temperature capability, such as polymers and metals, without significant heating of the substrates, and permit localized deposition of coatings as well. Finally, the millimeter wave source has been used in the efficient production of nanophase materials, metal, alloy and ceramic powders, via a modification of the polyol process. The millimeter wave source permits greatly accelerating the polyol process with the result of production of powders of smaller particle size and greater particle size uniformity. The results and implications of the wide range of these materials processing experiments will be discussed, together with some results of theoretical calculations and modeling of millimeter-wave interaction with materials.