Abstract: The concept and technical characteristics of the multiple-input multiple-output(MIMO) sonar are discussed in this paper.According to different diversities(the waveform diversity and the spatial diversity) adopted in sonar systems,the MIMO sonar is divided into two kinds: the MIMO sonar with co-located elements(utilizing the waveform diversity) and the MIMO sonar with widely separated elements(utilizing the spatial diversity).For the former,the advantages,such as more degrees of freedom(DOFs),flexible transmitting signal design and direct applicability of adaptive array techniques,and the disadvantage such as the transmitting array gain loss are discussed.For the latter,it is further put into two types-the MIMO sonar with transmitting/receiving diversity and the MIMO sonar with transmitting diversity only.Both types of MIMO sonars with widely separated elements are discussed and related to the multi-static sonars in this paper.