This book reviews the potential mechanisms in arbuscular mycorrhizas (AMs), in the hope that this can help arbuscular mycorrhizal fungi (AMF) to be more used efficiently as a biostimulant to enhance stress tolerance in the host plants. AMF, as well as plants, are often exposed to all or many of the abiotic and biotic stresses, including extreme temperatures, pH, drought, water-logging, toxic metals and soil pathogens. Studies have indicated a quick response to these stresses involving several mechanisms, such as root morphological modification, reactive oxygen species change, osmotic adjustment, direct absorption of water by extraradical hyphae, up-regulated expression of relevant stressed genes, glomalin-related soil protein release, etc. The underlying complex, multi-dimensional strategy is involved in morphological, physiological, biochemical, and molecular processes. The AMF responses are often associated with homeostatic regulation of the internal and external environment, and are therefore critical for plant health, survival and restoration in native ecosystems and good soil structure.