The micro-Doppler signature is a distinctive characteristic of the observed micro-Doppler effect in an object. The “signature” is commonly used to refer to the characteristic expression of an object or a process. When examining the micro-Doppler effect, the distinctive micro-Doppler characteristic, i.e., the micro-Doppler signature of an object, allows the recognition of an object's identity through its movement. To extract micro-Doppler signature of an object by radar, a simple continuous wave (CW) radar is good enough. However, wide-band coherent Doppler radars, which use wide frequency bandwidth to gain high range resolution and measure both the range and Doppler information, are more useful and desirable in the radar micro-Doppler signature research. In this chapter, we review current progress in the extraction, processing, analysis, and applications of radar micro-Doppler signatures, and discuss the challenges and perspectives in radar micro-Doppler signature research.