Ultra-wideband (UWB) radio signals employ the transmission of very short impulses of radio energy whose characteristic spectrum signature extends across a wide range of radio frequencies. Because UWB signals have high bandwidths and frequency diversity, they are particularly well suited for high-speed data communication in environments, such as indoors, where multipath fading is likely. UWB has other beneficial characteristics. The radio spectrum is considered to be fully utilized and, in fact, in short supply. By their nature, UWB signals utilize spectrum already designated for other use and regulated by the Federal Communications Commission (FCC). Emerging short-range radio applications, however, have recently encouraged the development of low-power emission systems and the low power density of these signals allows them to overlay present usage without harmful interference. Moreover, a UWB radio can be designed to be very low power, compact, and inexpensively integrated. Ultra-wideband radio, sometimes referred to as baseband, impulse or carrier-free radio, is not a new concept: Heinrich Hertz and others used spark gaps to generate UWB signals before sinusoidal carriers were introduced at the beginning of the last century. However, only recently has it been possible to efficiently generate and control UWB signals and apply modulation, coding and multiple access techniques to make UWB competitive for wireless communication applications. This paper contains a description of UWB, of its relevant characteristics and of its advantages with respect to other technologies for certain communication applications. Some of the techniques used to implement a high capacity, low cost spectrum efficient system, and some of the challenges facing the further development of this new technology, are also described.