In this chapter we present UWB communication as a potential candidate for cognitive radio technology. Cognitive radios are intelligent radios that could adopt itself by sensing and learning the radio environment and optimize its transmission strategies to maximize the utilization of the scarce radio resources such as the radio spectrum. This has been motivated by the radio regulatory bodies around the world (EC, 2007; FCC, 2003) to utilize unused radio spectrum known as white space in the spatio-temporal domain. In the recent years UWB communication has emerged as a potential candidate for the CR technology due to its ability to share the spectrum with others for short range wireless communications. In this context we present the concept of cognitive radios and the necessary techniques to adopt UWB as cognitive radios in this chapter. Especially, we enhance on the fundamentals of cognitive radios and spectrum sensing which enable the UWB radio to learn the radio environment. We also touch upon other cognitive radio related topics that are related to UWB communications such as dynamic spectrum access, interference mitigation and localization techniques. Furthermore, we present some potential applications for the use of UWB based cognitive radios which are derived from the European Union funded projects EUWB (EUWB, 2008) which is one of the biggest UWB projects that the world has seen so far, and the C2POWER project (C2POWER, 2010) which is related to energy efficiency in short range wireless communications with the use of cognitive radios. In this chapter we do not consider the technological aspects related to the use of cognitive radios for energy efficiency but only consider the use of cognitive radios for dynamic spectrum access. However, at the end of the chapter we present a scenario for the use of cognitive radios for energy efficiency derived from (C2POWER, 2010). In the material presented in this chapter we mainly consider the high data rate UWB radios based on the Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) technique following the Wimedia specifications (Wimedia-PHY, 2009). The OFDM based transceiver design makes it feasible for the UWB radio to sense the radio environment and dynamically change the transmission parameters accordingly. This makes the UWB 11