Ultra Wide Band (UWB) technology consists of emitting radio signals over frequency bandwidths from 500 MHz to several GHz. Its unique characteristics may be exploited for the design of high data rate wireless communication systems, as well as localization and imaging applications. The development and optimization of such systems require a precise knowledge of the radio transmission medium. This textbook examines all aspects of the propagation channel for UWB systems. UWB technology is first presented, with a particular emphasis being given on its applications, spectrum regulation issues and the different communication techniques. Next, the authors introduce the theoretical bases of radioelectric propagation and give an overview of the channel sounding techniques adapted for UWB signals. The two main principles of UWB channel modeling are finally exposed and illustrated: deterministic channel modeling, based on the simulation of the propagation phenomena in a given environment, and statistical channel modeling, which relies on the experimental analysis of the main channel characteristics.