Research has been conducted in wired network Intrusion Detection Systems (IDS) for over 25 years. Although there is ongoing research in wired IDS techniques, it is considered a mature technology. Wireless area networks and personal area networks have been the focus of recent research, as they represent new risks and security challenges. Mobile Ad Hoc Networks (MANETs) have further challenged researchers to develop IDS techniques in an even more challenging environment. The promise of wireless sensor network technology to provide cost-effective monitoring of critical applications ranging from industrial control to border monitoring necessitates new research in the area of wireless sensor network IDS. The unattended nature and the inherent computational and communication limitations of sensor networks make them vulnerable to a broad range of attacks. Given the relative infancy of this new technology, the limited documented cases of actual sensor network attacks, the lack of publicly available network traces of sensor network attacks, most experience in this area is limited to simulations or laboratory emulations, with few approaches having been vetted in the field. This chapter outlines the unique challenges of wireless sensor network IDS and provides a survey of solutions proposed in the research literature