146Wireless mesh networks are potentially vulnerable to a broad variety of attacks. Hence security is an important consideration for the practical operation of wireless mesh networks. Within security, intrusion detection is the second line of defense in wireless networks as well as wired networks. Unfortunately, wireless mesh networks present additional challenges due to their decentralized nature, dynamic network topology, and easy access to the radio medium. Due to these unique challenges, intrusion detection techniques cannot be borrowed straightforwardly from wired networks. New distributed intrusion detection schemes must be designed for wireless mesh networks. This chapter describes the basics of intrusion detection and gives a survey of intrusion detection schemes proposed for wireless mesh networks. The schemes share some common concepts, but differ in the details which are compared. This chapter describes the difficulties with each scheme and ongoing challenges. Due to the difficult challenges presented by the wireless environment, intrusion detection in wireless mesh networks is still an open research problem.