This chapter provides a comprehensive overview of the classification of the phylum Glomeromycota, a pivotal group of obligate symbiotic fungi known as arbuscular mycorrhizal fungi (AMF). These fungi are critical ecological partners of terrestrial plants, facilitating nutrient uptake and enhancing stress tolerance across various environments. With the advent of molecular phylogenetics—particularly rRNA gene sequencing—taxonomic clarity has significantly improved, enabling the delineation of five principal orders within Glomeromycota: Archaeosporales, Diversisporales, Gigasporales, Glomerales, and Paraglomerales. Each order is examined through the lens of spore morphology, symbiotic strategy, and genetic markers. Notably, Geosiphon pyriformis, the sole species in the family Geosiphonaceae, diverges from the classic AMF model by forming endosymbiosis with cyanobacteria rather than vascular plants, offering unique insights into ancient fungal symbioses. In contrast, typical AMF such as those in Glomeraceae and Gigasporaceae exhibit characteristic spore types—glomoid, acaulosporoid, and gigasporoid—and complex wall structures critical for taxonomic identification. The molecular distinctiveness of families such as Intrasporaceae and Ambisporaceae is emphasized through their conserved and diagnostic gene regions, including SSU, ITS, and LSU rDNA. Ecologically, these fungi demonstrate remarkable adaptability, inhabiting diverse soils, including saline and arid environments. Their contributions extend beyond nutrient mobilization to include disease suppression, soil structure improvement, and biodiversity maintenance. The chapter underscores the importance of integrating morphological, ecological, and molecular data to achieve robust taxonomic resolutions. The emergence of new families such as Intraornatosporaceae reflects ongoing revisions in AMF taxonomy, informed by next-generation sequencing and metabolomic analyses. These developments reinforce the dynamic and evolving understanding of Glomeromycota, which continues to challenge classical concepts of fungal systematics. Overall, this chapter serves as both a foundational and advanced resource for students, researchers, and professionals interested in mycology, symbiosis, and fungal biodiversity.