Mycorrhizae represent a symbiotic relationship between a fungus and plant roots. The fungus colonizes the roots of the host plant, either intracellularly in arbuscular mycorrhizal fungi, or extracellularly in ectomycorrhizal fungi. Host plants enhance the defense of mycorrhizal fungi against fungivores. Mycorrhizae mediate host plant-insect interactions. Arbuscular mycorrhiza play a crucial role in augmenting plant resistance to stressful conditions like diseases, drought, heavy metal toxicity, high temperatures, salinity, and waterlogging. They are also important in enhancing decomposition of toxic organic substances, in remediating polluted environments, and in keeping a stable ecosystem and sustainable productivity. This book discusses the structure, development and function of mycorrhizas.