During the past 30 years, numerous philosophical and procedural discussions have focused on the principles and methods of biological classification. From this have emerged three schools, phenetics, cladistics, and phyletics, each with its own theory of relationships, and its own set of explicit algorithms. At the same time, numerous studies on the economically important arbuscular mycorrhizal fungi have been completed. Some explicit approaches to classification have been made with higher fungi, and cladistic work has just begun on the vesicular arbuscular mycorrhizal (VAM) fungi. Thus it is now appropriate to examine the current status of the systematics of arbuscular mycorrhizal fungi and make suggestions for future work. The highest priority is to establish more refined circumscription of taxa at the lower levels of the hierarchy and to communicate these data and concepts to practicing physiologists, ecologists, etc., so that more rapid progress can be made in these areas. The limits of diversity of the group worldwide should be sought. A standardized identification manual (or atlas) would be most useful. Macromolecular studies are needed to help determine the close relatives of the VAM fungi as well as their interrelationships at the specific and generic levels. Continued emphasis needs to be placed on ontogenetic studies and their impact on variation in characters and states for taxonomic purposes. New quantitative classification approaches need to be used within the group, including phenetics and phyletics. Biogeographic concerns can then be addressed after the taxonomic issues have been more fully settled.