The arbuscular mycorrhizal (AM) fungi are the most common obligate symbiotic fungi, occurring on diverse group of plants from bryophytes to pteridophytes, gymnosperms and angiosperms in different ecological conditions. The AM fungi benefit the host plant by increasing the absorption capacity of the root system and translocate inorganic nutrients mainly phosphorus, other complexed compounds and minor elements through extensive network of hyphae to the external root more effectively than the plant roots. More recently AM fungi are transferred from the polyphyletic phylum Zygomycotina to newly erected monophyletic phylum Glomeromycota. This new phylum Glomeromycota is classified into 3 classes, 5 orders, 14 families and 29 genera. The nutrient status of the soil, pH, OC and host cultivars will influence the AM fungal population and root colonization. The book will help graduate, post graduate students and research scholars as a ready reference. Differences in spore development, spore ornamentation, spore wall layers, root colonization patterns and molecular studies are some of the major diagnostic features that can be used in distinguishing different genera of Glomalean fungi.