Millimeter-wave plays an indispensable role in the new generation of mobile communication because of its abundance of unexplored resources, which can be used to meet the requirements of greater bandwidth and ultra-high data rate. This chapter firstly introduces the development, characteristics, and applications of millimeter-wave. Then the application of millimeter-wave in mobile communication systems is described in detail, and the common channel model is listed. Secondly, a large-scale MIMO system, precoding technology, and three kinds of beam forming structures are introduced. Finally, combined with the current development of mobile communication, the requirements on the millimeter-wave devices for the new generation of mobile communication system are summarized, and some typical millimeter-wave devices are listed.