Wireless sensor networks have attracted high interest over last few decades in the wireless and mobile computing research community. Applications of wireless sensor networks are numerous and growing, including indoor deployment scenarios in the home and office to outdoor deployment in adversary⁰́₉s territory in a tactical background. However, due to distributed nature and their deployment in remote areas, these networks are vulnerable to numerous security threats that can adversely affect their performance. This problem is more critical if the network is deployed for some mission-critical applications such as in a tactical battlefield. Random failure of nodes is also very likely in real-life deployment scenarios. Due to resource constraints in the sensor nodes, traditional security mechanism with Lange overhead of computation and communication is infusible in wireless sensor network. Design and implementation of secure wireless sensor networks is, therefore a particular challenging task. This book covers a comprehensive discussion on the state of the art in security technologies for wireless sensor networks. It identifies various possible attacks at different layers of the communication protocol stack in a typical wireless sensor network and presents their possible countermeasures. A brief discussion on the future direction of research in wireless sensor network security is also included