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　fee　movement　of　mobile　nodes　can　lead　to　ftequent　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.