The possible role of mycorrhizae in the biological control of root pathogens, biological nitrogen fixation, hormone production, and drought resistance attracted workers from different disciplines. The vesicular-arbuscular (VA) mycorrhizal association is found in most plant families so far examined, although it may be rare or absent in families such as Cruciferae, Chenopodiaceae, Caryophyllaceae and Cyperaceae. The importance of VA mycorrhizae (VAM) fungi for plant growth in most soils has been accepted and acknowledged by soil chemists, agronomists, horticulturists, and farmers. VAM are formed by nonseptate phycomycetous fungi belonging to the genera Glomus, Gigaspora, Acaulospora, and Sclerocystis in the family Endogonaceae. VA mycorrhizal inoculation should soon be accepted as a part of standard nursery practice in raising containerized seedlings or cuttings in sterilized mix, or unsterilized mix with low or inefficient strains of mycorrhizal fungi. The chapter also presents an overview of the key concepts discussed in this book.