It has been a revelation that, strictly speaking, most plants do not have roots but rather mycorrhizae, a fact that has had tremendous consequences on the life of plants and the evolution of soil-plant systems. The research on arbuscular mycorrhizal (AM) symbioses has been intensive over the past forty years and we have learned a lot on the physiology, biology, ecology, and genetics of the symbiosis and the fungi involved in it. Most important, it appeared that cropping systems could be more sustainable with the management of AM fungi and reduced reliance on agrochemicals. The extraradical mycelia of AM fungi are an essential link between the plants, which are the consumers, and the soil, which is the provider. They are key organs enhancing plant uptake of nutrients, particularly phosphorus in high P-fixing soils, and consequently reducing crop dependence on fertilizers. They also improve soil quality. Thus, the nature of AM extraradical mycelia must be considered in the design of cropping practices that optimize the contribution of AM fungi to crop production. The nature and role of AM mycelia as plant providers are discussed in Chapters 1 and 2. How AM fungi reduce disease incidence in plants has not been clarified, but what appears clear from the extensive literature review presented in Chapter 3 is that AM fungi do provide an important level of bioprotection to plants. All research efforts on the study of AM do not translate into biotechnologies for agriculture and forestry in all parts of the world. In developed countries, the availability of agrochemicals at prices that farmers can afford has limited AM-related biotechnologies almost exclusively to soilless horticultural production. Arbuscular mycorrhizal inoculation has more impact in soilless than in field systems where native AM fungi are present. Chapter 4 reports on how AM fungi could be best used in horticultural production. Incentives to increase fertilizer use efficiency were larger in countries with weaker economies. This was true in Cuba where an important research group has been successful in developing better practices for crop inoculation with effective AM strains since the early 1990s, as reported in Chapter 5. Chapter 6 presents numerous reports, some of which are difficult to access directly, indicating that AM biotechnologies would be advantageously applicable in a large number of tropical crops. Plant nutrition and health and soil quality benefit from AM in tropical settings. Chapter 7 summarizes the mycorrhizal research conducted in India, where the use of AM inoculants is rapidly expanding. India has been a leader in the development of AM technologies for crop production, whereAMinoculants are used not only in crop production but are also very useful in soil rehabilitation. Evidence of negative impacts of human activities, including crop production, on the environment and climate of the Earth are presented in Chapter 8, in a warning call reestablishing the need for AM agricultural research and development in wealthier countries. This bookwas prepared to serve as the basis for a second round of research efforts to improve cropping systems’ sustainability throughout the world.