Massive MIMO has been regarded as a promising technique for 5G wireless communication networks. It equips the base station (BS) with a large number of antennas to serve a set of user equipment (UEs) simultaneously. Due to channel hardening, the uncorrelated noise and the small‐scale fading can be eliminated. As a result, the transmitted signal can be directed into very focused areas, which brings orders‐of‐magnitude improvements in both spectral efficiency and energy efficiency with simple (linear) processing. In this chapter, we present a comprehensive overview of the state‐of‐the‐art research on massive MIMO. We begin with an information theoretic analysis to illustrate the advantages of massive MIMO. Then, we introduce both the physical layer and networking issues of massive MIMO. The integration of other 5G technologies with massive MIMO is also reviewed. Finally, we conclude this article with a discussion of challenges and future research topics.