Ultra-wideband (UWB) technology provides an excellent means for wireless positioning due to its high resolution capability in the time domain. Its usability to resolve multipath components makes it possible to obtain accurate location estimates without the need for complex estimation algorithms. This precise location estimation capability facilitates many applications such as medical monitoring, security, and asset tracking. This paper discusses the theoretical limits of TOA estimation and TOA-based location estimation for UWB systems. Due to the complexity of the optimal schemes, suboptimal but practical alternatives are emphasized. Performance limits for hybrid TOA/SS and TDOA/SS schemes are also considered.