As a new type of radar, multiple-input multiple-output (MIMO) radar exploits multiple transmit-antennas which transmit different waveforms to obtain more degrees of freedom and angular diversity gain and hence combat target radar cross section (RCS) scintillation and improve radar performance in target detection, multiple targets identifiably and parameters estimation. The desired transmit beam pattern can be formed much more flexibly by optimization of the transmitted waveform according to actual application. This paper provides a general introduction to the origin, development, current state, dominant types and characteristics of MIMO radar as well as the main research directions, the research institutes and their works.