An　Ad　Hoc　network　is　a　multi-path　autonomous　system　comprised　of　a　group　of　mobile　nodes　with　wireless　transceivers.　In　this　paper,　we　first　review　the　present　research　in　key　management　in　Ad　Hoc　networks.　Then,　by　considering　the　characteristics　of　Ad　Hoc　networks　in　which　the　free　movement　of　nodes　can　lead　to　a　variety　of　topological　changes,　especially　network　separation　and　convergence,　we　propose　a　new　key　management　scheme　based　on　a　combination　of　techniques,　such　as　hierarchical　topology　structure,　virtual　CA　(certification　authority),　offline　CA　and　certificate　chain.　We　show　that　the　proposed　scheme　would　improve　key　management　in　security,　expandability,　validity,　fault　tolerance　and　usability.