The rapid proliferation of wireless networks and mobile computing applications has changed the landscape of network security. As the recent denial-of-service attacks on several major Internet sites have shown us, no open computer network is immune from intrusions. The traditional way of protecting networks with firewalls and encryption software is no longer sufficient and effective. The wireless ad-hoc network is particularly vulnerable due to its features of open medium, dynamic changing topology, cooperative algorithms, lack of centralized monitoring and management point, and lack of a clear line of defense. Many of the intrusion detection techniques developed on a fixed wired network are not applicable in this new environment. How to do it differently and effectively is a challenging research problem. In this report, the vulnerabilities of a wireless ad-hoc network are examined. The reason why we need intrusion detection and the reason why the current methods cannot be applied directly are also explained. A study is also made on the new architecture for intrusion detection in mobile networks and the fundamentals of anomaly detection.