Recent research has indicated the importance of arbuscular mycorrhizal fungi (AMF) in mediating plant coexistence. Coarse-scale studies compare the effects of the presence versus absence of AMF on plant coexistence, a phenomenon that is most relevant in early successional ecosystems where AMF are patchily distributed. By contrast, fine-scale studies investigate interactions that might occur once AMF have developed more fully within ecosystems, and most plants come into contact with AMF. Whereas coarse-scale effects are well understood, our understanding of fine-scale factors is just developing, as a result of investigations into AMF-plant specificity, AMF species richness, shared mycelial networks, and plant-AMF feedback effects. Further research into these areas will provide a better understanding of factors that mediate plant species co-existence and, ultimately, the maintenance of biodiversity within plant communities.