Fifth-generation wireless technology, denoted as “5G” by the global wireless communications industry, seeks to accomplish multi-fold increases in key aspects of wireless communication. In conjunction with conventional microwave frequencies, millimeter-wave (mmWave) and THz frequencies and technologies are being investigated by major research institutions and industry. Deliberations are underway by regulatory officials around the world to authorize mmWave and sub-THz spectrum for 5G fixed and mobile applications. In the development of 5G communication systems, new propagation phenomena, novel multi-antenna transmitting architectures, and new mmWave operating frequency bands using much wider channel bandwidths than ever before are proving to be extremely effective ways to dramatically increase the communication data rate of future mobile communication networks. The papers in this special issue offer extensive insights and promising results that address many of the technical challenges that must be solved to usher in the 5G era.