Ultrawideband (UWB) communication is becoming more attractive for low-cost personal communication applications, due to its various advantages, such as the unlicensed usage of an extremely wideband spectrum and the potential for achieving very high data rates. Since UWB wireless communication systems share their spectra with many Iicensed narrower-bandwidth radios, the permitted power spectral density of UWB signals is rather limited, due to the regulations imposed by relevant authorities, such as the Federal Communications Commission (FCC) in the USA and the European Commission (EC) in Europe. These regulations place significant restrictions upon the data rates and communication distances that the UWB systems can achieve. One possible way to solve this problem is to employ relays. This dissertation focuses on the design and performance of relay-assisted UWB systems. First, single- and multiple-antenna relaying schemes are developed for UWB systems in order to enhance their coverage and reliability. Compared with the single-antenna relaying schemes, the multiple-antenna relaying schemes can further improve the system performance. Then, an advanced UWB relay system using multiple relays, each of which is equipped with multiple antennas, is presented. Increasing the number of antennas is found to be more beneficial to the system performance than increasing the number of relays. Next, one-way and two-way UWB relay systems using a multiple-antenna relay with antenna selection, as weil as those using single-antenna relay selection, are proposed. For most of these proposed systems, using the multiple-antenna relay with antenna selection generally provides better performance than using the singleantenna relay selection. Furthermore, the design and performance analysis of various UWB multiple-input multiple-output (MIMO) relay systems are presented, in which the source, relay, and destination have multiple antennas. The results are promising, and suggest the marriage of MIMO and relaying in the UWB context. Finally, the measurement results for UWB relay channels in indoor environments are presented and, based on such results, the performance of the designed MIMO relay systems is evaluated. This provides useful insights into the performance of UWB MIMO relay systems in real UWB channels.