Mycorrhizae are indigenous to soil and plant rhizosphere and potential tools for sustainable agriculture. Endomycorrhizae are symbiotic root-fungus associations, formed by nearly more than 80% of terrestrial plants. These symbioses are characterized by bi-directional movement of nutrients where carbon flows to the fungus and inorganic essential nutrients move to the plants, thereby providing a critical linkage between the plant root and soil. As a result, endomycorrhizal plants are often more competitive and better able to tolerate environmental stresses than are non-mycorrhizal plants. In this connection, these symbioses play a key role in natural and agricultural ecosystems through major functions in the enhancement of plant phosphorus and other nutrients, soil fertility & conservation, water economy, alleviation of salt stress, reducing the heavy metal toxicity and biocontrol of pathogenic soil microorganisms.In this book we attempt to present new strategies that might increase functions of these endomycorrhizal fungi with the associated economic plants.This includes the incorporation of specific genes known to up-regulate the VA mycorrhizal interactions with their plant hosts.