Demand for wireless throughput, both mobile and fixed, will always increase. One can anticipate that, in five or ten years, millions of augmented reality users in a large city will want to transmit and receive holographic video more or less continuously, say 100 megabits per second per user in each di- rection. Massive MIMO—also called Large-Scale Antenna Systems—is a promising candidate technology for meeting this demand. Fifty-fold or greater spectral efficiency improvements over fourth generation (4G) technology are frequently mentioned. A multiplicity of physically small, individually controlled antennas performs aggressive multiplexing/demultiplexing for all active users, utilizing directly measured channel characteristics. By leveraging time-division duplexing (TDD), Massive MIMO is scal- able to any desired degree with respect to the number of service antennas. Adding more antennas is always beneficial for increased throughput, reduced radiated power, uniformly great service everywhere in the cell, and greater simplicity in signal pro- cessing. Massive MIMO is a brand new technology that has yet to be reduced to prac- tice. Notwithstanding, its principles of operation are well understood, and surprisingly simple to elucidate.