Massive MIMO is widely recognized as an essential technology for 5G. Together with newly allocated spectrum (bandwidth) and network densification (small cells), it is expected to play a key role in coping with the ongoing explosion in data-traffic demand and services. Compared to 4G MIMO technologies, massive MIMO can offer large gains in cell spectral efficiency, which, in combination with small cells and additional bandwidth, can translate into vast gains in throughput per unit area. We briefly overview the most promising TDD and FDD operation modes for massive MIMO, and discuss their potential benefits and challenges considering operation over different tiers and frequency bands. TDD operation is naturally suited to massive MIMO and can offer “massive MIMO” gains, with simple in-cell processing, low overheads and low end-to-end latencies. We also briefly describe some important massive MIMO activities towards 5G, including standardization efforts, system development and experimental trials.