A mobile Ad Hoc network is a multi-path autonomous system comprised of many mobile nodes with wireless transmission capability. In this paper, we first review current research in intrusion detection in Ad Hoc networks. Then, by considering the characteristics of such networks in which free movement of mobile nodes can lead to frequent topological changes, especially network separation and convergence, we propose an intrusion detection scheme based on a combination of techniques, such as hierarchical topology structure, distributed voting, off-line CA and composite key management, and show that the proposed scheme could improve intrusion detection in the areas of security, expandability, validity, fault tolerance and usability.