This thesis puts a focus on the analysis of key aspects of low-complexity Ultra Wideband (UWB) localizations systems. It is well known that UWB allows for highly robust and accurate ranging even in multipath intensive environments. On the other hand, the huge bandwidth leads to very challenging receiver designs and so low complexity and low power consumption are not achieveable for common receiver structures. The energy detector is a promising alternative. But in contrast to high-complexity coherent re- ceivers, their performance is strongly dependent on the system parameters of the air interface protocol. IEEE 802.15.4a is a UWB standard with high-precision localization capability (better than 1m). The standard defines many system parameters, whose impact on the ranging and localization performance is studied in the thesis. These parameters have also a significant impact on the maximum allowed transmit energy, which limits the operating range of the localization system. Thus, the maximum operating distance is analyzed for energy detectors and coherent receivers. The analysis is based on a link budget according to the FCC/CEPT regulations and on statistical models of the receiver structures. A UWB demonstrator system has been developed for ranging and positioning experiments. The IEEE 802.15.4a standard has been implemented and the system satisfies the FCC/CEPT regulations. The processing is held offline to achieve high flexibility. In this work, the demonstrator system is used to measure channel impulse responses in an extensive measurement campaign in indoor and outdoor environments. These measurements are used to study inter-pulse-interference (IPI) of energy detectors in IEEE 802.15.4a. The channels are characterized with the parameterization of a pathloss model and their root mean square (RMS) delay spread. Finally, a system-level simulator for positioning and tracking (U-SPOT) is presented to evaluate the influence of system parameters and algorithms on the overall performance of a localization system. The simulator is based on the other outcomes of the thesis, where the measurements, channel models, link budgets, and the receiver structures are combined to form a novel statistically defined simulation framework. In this work, U-SPOT is used to study the influence of NLOS links on the localization performance of IEEE 802.15.4a. Sub-meter accuracies are achieved by both receivers in LOS and NLOS situations. That is, energy detectors are suitable for low-complexity localization systems. Surprisingly, ranging based on IEEE 802.15.4a is possible up to several thousands of meters for coherent receivers. Even for the low- complexity energy detector, up to several hundreds of meters are achieved. A longer spacing of the pulse sequences leads to two advantages: First, it reduces inter-pulse-interference and second, more transmit power is allowed by the FCC/CEPT regulations, which leads to longer operating distances.