Recent advances in mobile device, wireless networking, and positional technologies have helped location-aware applications become pervasive. However, location trajectory privacy concerns hinder the adoptability of such applications. In this article, we survey existing trajectory privacy work in the context of wireless sensor networks, location-based services, and geosocial networks. In each context, we categorize and summarize the main techniques according to their own feathers. Furthermore, we discuss future trajectory privacy research challenges and directions.