Wireless technologies have transformed civilization providing an unprecedented level of connectivity. The wireless signals that propagate through space not only can carry information over a distance, but can also reveal information about the propagation paths. The later has enabled radio-based localization, where the signal’s propagation time, phase, and distortions inform the distance and angle, and describe the surroundings between transmitter and receiver pair, which are primitives for finding location. This thesis focuses on indoor localization using wireless signals. We study the methods and algorithms of indoor localization, with an explicit attention to problems in practical systems, including privacy, scalability, robustness, security, and usability. We conduct analysis on the errors in distance measurement due to imperfect hardware, and create a tool for easy comparison and visualization of ranging protocols. We demonstrate a system that, with carefully designed localization methods, can not only achieve good accuracy, but also support a large number of users, protect user privacy, cover large areas, and require low deployment effort. We investigate the range spoofing vulnerability in the widely adopted localization scheme, and propose a system to detect and counter such an attack. Finally, we demonstrate a robust cryptographic key generation solution based on wireless propagation paths that provides additional security safeguard to localization-enabled services.