This monograph discusses the structural matching of description logic ontologies. Our belief is that structural matches of ontologies hold some deeper information about the similarities between different ontologies than a mere syntactic match. Ontologies are parsed into attributed trees where nodes represent the concepts and edges represent the relationship between concepts. T h e match between any two nodes is calculated by traversing the derived trees in an efficient algorithm that limit the number of comparisons to be performed by eliminating unnecessary comparison in order to maintain a polynomial running time. For each pair of node, an aggregated weight that defines the level of the match is evaluated by using the level of similarity of their parents and those of their successors. The obtained weights are finally used to predict the similarities between concepts and consequently, that of ontologies.