The wireless technologies are bringing significant changes to data networking and telecommunication services, making integrated networks a reality. By removing the wires, personal networks, local area networks, mobile radio networks, and cellular systems, offer an entirely distributed mobile computing and communications environment. Due to their unique features such as shared medium, limited resources, and dynamic topology, wireless ad hoc networks are vulnerable to a variety of potential attacks. However, the common security measures employed for wired networks are not enough to protect the nodes of the networks against complex attacks. Therefore, a new line of defense, called intrusion detection, has been added. In this chapter, first we introduce the main wireless technologies along with their characteristics. Then, a description of the attacks that can be mounted on these networks is given. A separate section will review and compare the most recent intrusion detection techniques for wireless ad hoc networks. Finally, based on the current state of the art, the conclusions, and major challenges are discussed.