Ultra-Wideband (UWB) has recently gained great interest for high-speed short-range communications (e.g. Wireless Sensor Networks/home networking applications) as well as low-speed longrange communications as it can offer unique advantages for wireless communications such as precise location-timing capabilities, imperceptibility, low power, low complexity, and low cost. In this chapter, we present an overview of UWB system and a performance analysis of UWB based MAC protocol of high data rate, short range communication, especially, on the IEEE 802.15.3 MAC protocol for wireless sensor networks. We use analytical model to present some illustrative channel capacity results, and provide a discussion of the impact of various acknowledgment schemes on multiple access capacity under different parameters. Also, some important observations are obtained, which can be very useful to UWB MAC protocol architectures as well as to UWB application designers. Finally, we present some important research issues and challenges to further investigate the possible improvements in the UWB system.