Demand for wireless communications is projected to grow by more than a factor of forty or more over the next five years. A potential technology for meeting this demand is Massive MIMO (also called Large-Scale Antenna Systems, Large-Scale MIMO, ARGOS, Full-Dimension MIMO, or Hyper-MIMO), a form of multi-user multipleantenna wireless which promises orders-of-magnitude improvements in spectral-efficiency over 4G technology, and accompanying improvements in radiated energy-efficiency. The distinguishing feature of Massive MIMO is that a large number of service-antennas - possibly hundreds or even thousands - work for a significantly smaller number of active autonomous terminals. Upsetting the traditional parity between service antennas and terminals in this manner is a game-changer: The simplest multiplexing pre-coding and de-coding algorithms can be nearly optimal, expensive ultra-linear forty-Watt power amplifiers are replaced by many low-power units, and the favorable action of the law of large numbers can greatly facilitate power-control and resource-allocation. Massive MIMO is still an emerging field. There are many unanswered theoretical questions and much remains to be done to obtain a reduction to practice. The six papers in this Special Issue are a sampling of the types of problems that are topics of active research. The papers logically fall into three categories: a) Acquisition of Channel State Information, b) Spatial Multiplexing Algorithms, and c) Massive Array Issues and Architectures.