Insects and their hostplants represent the major part of terrestrial diversity, yet we are just beginning to understand why there are so very many species. By far the most influential model of insect/plant diversification has been Ehrlich and Raven's (1964) hypothesis of insect/plant coevolution. While the coevolution model was based on macroevolutionary patterns in plant defenses and hostplant affiliations, most of the subsequent work has been on its possible ecological and genetic mechanisms, with relatively little systematic scrutiny of the evolutionary patterns Ehrlich and Raven described. We explore the possible roles insect/plant interactions may play in the long-term evolution of insect and plant lineages, and review some of the evidence on whether or not insects and plants have exerted reciprocal influences on each other's diversification.Insects and plants have diversified over roughly the same time intervals, and many insect host-affiliations are evolutionarily conserved, thus reflecting long-term, phylogenetic history. Rather than accumulating herbivores at a rate proportional to their geographic area of distribution or biomass, some plant groups pose apparent chemical barriers to potential herbivore colonists, and seem accessible to relatively few insect lineages, possibly preadapted by use of chemically similar or related hostplants. Evolutionary innovations in plant defenses and in insect feeding habits seem to have spurred their respective adaptive radiations, thus ecological opportunity may influence long-term evolutionary success. The greater diversity of insects and plants in the tropics, compared to the temperate zone, probably reflects the greater age of tropical habitats as well as climatic barriers that limit successful invasion of the temperate zone to just those primitively tropical groups able to evolve strategies for both over-wintering and use of temperate resources. Though evidence is still sparse, successful invasion of the temperate zone may promote subsequent radiations of both insects and plants.We conclude that much of the available evidence from systematics is consistent with Ehrlich and Raven's suggestion that much of insect and plant diversification has been spurred by a series of ecological opportunities over evolutionary time.