Rapid technological advances in the area of Micro Electro-Mechanical Systems (MEMS) have spurred the development of small inexpensive sensors capable of intelligent sensing. A significant amount of research has been done in the area of connecting large numbers of these sensors to create robust and scalable Wireless Sensor Networks (WSNs). Proposed applications for WSNs include habitat monitoring, battlefield surveillance, and security systems. Although individual sensor nodes have limited capabilities, WSNs aim to be energy efficient, self-organizing, scalable, and robust. A substantial amount of research has centered on meeting these challenges, but relatively little work has been done on security issues related to sensor networks.