Hello, Take this snippet: // g is an adjacency_list // u,v are vertices bool b; edge_desc e; tie(e,b)=boost::edge(u,v,g); if(b==false) std::cerr << "No such edge!"; boost::edge() is written to handle non-existent vertices: //abbreviated excerpt from detail/adjacency_list.hpp inline std::pair<edge_descriptor,bool> edge_dispatch(...) const { bool found; typename Config::OutEdgeList::const_iterator i = g.out_edge_list(u).find(StoredEdge(v)), end = g.out_edge_list(u).end(); found = (i != end); if (found) return std::make_pair(edge_descriptor(u, v, &(*i).get_property()), true); else return std::make_pair(edge_descriptor(u, v, 0), false); } However, I don't believe out_edge_list handles invalid vertices. If I provide an invalid source vertex, u, I get a segmentation fault on the call to g.out_edge_list(u).find(v). Thanks, -TAG Todd A. Gibson