The Ultra-Wide Band (UWB) technology consists in transmitting radio signals over a wide frequency bandwidth, typically in the order of 500 MHz to several GHz. This specificity can be exploited to develop high data rate, indoor communication systems. For the design and assessment of such systems, an accurate knowledge of the propagation conditions is necessary. This paper presents a number of channel sounding techniques that can be deployed for this purpose. A focus is made on MIMO channel sounding, involving multiple antennas at both the transmitter and receiver. Experimental characterization of the UWB channel reveals a strong influence of both the distance and frequency on propagation losses. The high temporal resolution leads to a channel impulse response presenting a large number of discrete propagation paths, in addition to diffuse scattered signal. The paper details the statistical SISO UWB channel models developed within the IEEE 802.15 standardization group, and discusses the extension of statistical models to MIMO systems.