100–300GHz wireless systems can provide very high data rates per signal beam, and, given the short wavelengths, even compact arrays can contain many elements, and hence can simultaneously transmit, in the same frequency band, many simultaneous independent signal beams to further greatly increase capacity. We will describe representative system designs, including wireless hubs and wireless backhaul links using massive spatial multiplexing, plus imaging radar systems, evaluate their feasible performance, and identify the key challenges in implementation, including transistor and IC performance, array physical design, digital beam former complexity, and systems cost.