The coevolution of arbuscular mycorrhizal fungi and most of today's plant lineages is considered in this chapter to have evolved twice, with most knowledge of ancestor properties and age confined to the lineage now classified as the suborder Glomineae (genera Glomus, Sclerocystis, Acaulospora, Entrophospora). Virtually nothing is known about evolutionary origins of the Gigasporineae lineage (genera Gigaspora, Scutellospora) since neither fossils nor extant fungi in other groups that might be related have been found. In both lineages, parallel function suggests similar coevolutionary trends. Speciation is considered to have been promoted in both symbionts following integration of the AM association, with the disproportionately greater number of species in the phytobiont being a function of morphological potential (complexity in design and life history traits) rather than any ecological causation. Ecology may be an important determinant of rate of speciation, but I propose that its central role has been in gradual coaccommodation and optimization of both symbionts as they changed and adapted in spatially and temporally complex communities. Universal mechanisms-or models to explain mycorrhizal phenomena are expected to be rare because the symbiosis was established so early that the mycobiont and phytobiont evolved together and remained integrated together during their coevolution to the present day. The evolutionary stability of the AM symbiosis indicates a general positive outcome for the phytobiont, since it is the higher unit of selection (and hence regulator) of the symbiosis. However, the causal processes for mutualism are as diverse as the magnitude of genealogical divergence among phytobiont lineages, and extend far beyond the growth benefit so widely touted but so inconsistently expressed in nature. The mycobiont appears to be more homogeneous in structure and function, but accumulating evidence indicates considerable potential for divergence in behavior and physiology within the constraints imposed by the phytobiont. This divergence is a function of genealogy, coevolutionary pressures, and coadaptation.