ABSTRACT
The natural and easy concept of subgraphs has become most fruitful in graph theory. One reason for this is that numerous and important classes of graphs are characterized by forbidden subgraphs. Let us just recall that forbidden odd cycles as subgraphs characterize bipartite graphs, that the subgraphs K5 and K3,3 mark the origin of topological graph theory, and that we have used the characterization of outerplanar graphs by forbidden subgraphs in Chapter 4.
