Theory and applications of Ultrawide Band (UWB) radiosystems which belong to a new intensively developing area in radar and communications are considered in the paper. Main distinguishing features of UWB radiosystems are systematized, while difference between such systems and traditional narrow-band ones is analyzed. Scientific and technical challenges faced by UWB radar designers are shown. Some new technical and circuit solutions used in UWB radiosystems are considered on examples of UWB radars. Possible directions in development of UWB technology and technique for such systems are suggested.