Abstract: Metal additive manufacturing is an emerging manufacturing technique over the past 30 years. Different from the traditional subtractive manufacturing technology, metal additive manufacturing is based on the principle of discrete-stacking and is in fact a layer-by-layer processing to obtain three-dimensional structures, according to three-dimensional model generated by the computer-aided design. Metal additive manufacturing has the advantages of near net-shaping, rapid manufacturing, and high design freedom. Therefore, it is very suitable for the direct forming of high melting point metal materials and structures with complex structures. Metal additive manufacturing has huge technical advantages and broad application prospects in aerospace, nuclear industry, automotive industry, and biomedical engineering. We first briefly introduce the principles of three typical metal additive manufacturing technologies, including selective laser melting, laser metal deposition and selective electron beam melting. We also summarize their research advances and their differences. Then, we review the recent advances in the formation mechanisms and control methods of defects (such as lack of fusion, pores, and cracks) in metal additive manufacturing. We also emphasize the influences of process parameters (such as laser power, scanning speed, and scanning strategy) on the microstructures of metallic materials fabricated by metal additive manufacturing. We further summarize the printable materials (including traditional alloys, high-entropy alloys, and metallic glasses) and their mechanical properties and performances. Finally, we point out some open issues and challenges for future research, including the expansion of the printable alloy systems, the quantification of the influences of defects and residual stress on mechanical properties, the development of simulation methods to predict the microstructures of metallic materials produced by metal additive manufacturing, and the establishment of relevant databases and standards.