Abuscular mycorrhizal (AM) fungi are ubiquitous in soils around the world where they form symbiotic associations with the majority of plant species. The beneficial effects of AM fungi are well known. They include improved plant nutrient uptake, enhanced N2-fixation in legumes, increased plant tolerance to biotic and abiotic stress and improved soil structural quality. They are seen as the cornerstone of sustainability in agro-ecosystems. This book presents current research in the study of mycorrhizal fungi.