Recently, a novel conception of Synthetic Aperture Radar (SAR) based on Multi-Input Multi-Output (MIMO) technology draws much attention for its potential advantages. MIMO-SAR could obtain much more equivalent channels than the number of the physical array elements by simultaneously utilizing multiple antennas at transmission and reception. These additional channels are demonstrated to be useful for the application of High-Resolution Wide-Swath (HRWS) imaging and slowly moving target indication. In this paper, a detailed discussion on the conception and connotation of MIMO-SAR is made firstly, and then the investigation states of MIMO-SAR, such as high range resolution SAR imaging, three-dimensional down-looking SAR imaging, HRWS imaging and Ground Moving Target Indication (GMTI), are discussed. Base on the discussion mentioned above, the advantages and disadvantages of MIMO-SAR system are analyzed, and the key technical issues in MIMO-SAR are summarized. At last, the prospects of MIMO-SAR application are pointed out.