This chapter discusses some of the physiological findings with respect to the vesicular-arbuscular mycorrhizal (VAM) symbiosis and elucidates some of the implications for the fungal and plant partners of the association. Various auxins, cytokinins, gibberellins, and vitamins are produced by ectomycorrhizal fungi in pure culture, although. The effects of the fungus on host nutrition and carbon metabolism are also likely to be influenced by the proportion of the fungus which is metabolically active. The enhanced phosphorus uptake by mycorrhizal plants is most likely to be due to the external fungal hyphae acting as an extension of the root system, thereby providing a more efficient absorbing surface for uptake of nutrients from the soil and for translocation to the host root. Some phosphate-solubilizing bacteria and soil fungi can release phosphorus into the soluble phosphorus pool of the soil, and mycorrhizae might utilize this extra source.