Ideas and rules on the systematics of parasite groups, and to a lesser extent their hosts, have been influenced by the assumption that coevolution between parasites and hosts has resulted in parallel traits in their phylogenies, which, in turn, may be reflected in their systematics. The essays in this volume include studies of plant, insect, and vertebrate parasite groups and their hosts. The authors relate their findings to the implications of coevolutionary phenomena for the systematics of the organisms involved, and offer conclusions on the applicability of parasitological rules.