Complete explanations of mycorrhizal phenomena require knowledge of phylogenetic relationships among the fungal as well as the plant symbionts. Such interpretations derive from systematic analyses, which in turn depend on a strong taxonomic data base. Glomalean taxonomy still is in the formative stages of exploration and documentation of fungal diversity, but it is undermined by inattention to the biological properties of an unwieldy fungal organism and an inability or unwillingness to obtain living germplasm for comparative analyses. Few published descriptions are accurate and the type method in its present form has failed to provide a fixed reference point for restudy. New discoveries are in danger of being redundant and inconsequential unless a high priority is placed on reevaluation of known taxa. Systematic studies also are jeopardized by taxonomic deficiencies. Yet a phylogenetic perspective is essential to understand the fungal organism, define population and species concepts, formulate speciation and biogeographic process theories, and restructure classification to reflect evolutionary trends. A phylogeny is in place as a tool for making rigorous morphological, ontogenetic, ecological, physiological, and molecular comparisons. The International Culture Collection of Arbuscular and Vesicular-arbuscular Mycorrhizal Fungi (INVAM) is a resource that can provide culturable homogeneous germplasm for these kinds of experiments, nonliving specimens as known references for taxonomic investigations, and combinations of materials to aid in the understanding of the scope of fungal diversity. As causal connections between patterns of fungal diversification and mycorrhizal phenomena become known through all of these efforts, research results in systematics and other disciplines can be unified and incorporated into more universal theories.