The authors describe the existing WSN applications and discuss the research efforts conducted in this field. The authors explore the state-the-art protocol for WSN protocol stack in the transport, routing, data link and physical layers. In addition, the problem of synchronization and localization in WSNs investigated together with the existing solutions. Finally, the development of WSNs including sensor-actor networks, multimedia sensor networks, and WSN applications in underwater environments and underground exploration. It gives an overview of existing evaluation approaches to WSNs, including physical testbeds and simulation software environment. "Wireless Sensor Networks" is an important book for advanced students on courses in wireless communications, networking and computer science.