With the rapid development of wireless communications and the popularization of smartphones, higher data rate of the service is required. The demand can be efficiently and conveniently satisfied by increasing the number of antennas at base station (BS), thus resulting in a massive MIMO system. The massive MIMO system can exploit the spatial degree of freedom, and therefore BS is enabled to simultaneously serve several user equipments (UE) with the same temporal and frequency resource. Three key techniques are presented in the massive MIMO system, including the pilot contamination issue and the corresponding solutions, the theoretical and practical channel model that fits for the massive MIMO system, and the low complexity transmission and implement methods. Compared with the existing MIMO system, the massive MIMO system can distinctively increase the spectral efficiency, the energy efficiency, and the system robustness. As one of the most promising topics on the fifth generation (5G) of wireless communication systems, the massive MIMO system has attracted the extensive attention worldwide. Nevertheless, the research of massive MIMO systems is still at its early stage, with a lot of technical problems to be solved.