The rapid progress of wireless communication, embedded micro-electro-mechanical systems (MEMS), and microsensing technologies has made wireless sensor networks (WSNs) possible. A WSN consists of many inexpensive wireless sensors, which are capable of collecting, storing, processing environmental information, and communicating with neighboring nodes. In the past, sensors were connected by wired networks. With the development of ad hoc networking technologies, tiny sensors can communicate through wireless links in a more convenient manner. Much research has been dedicated to this area, including energy-efficient media access control (MAC) protocols, routing and transport protocols, self-organizing schemes, sensor deployment and coverage issues, and localization algorithms.