The definition of symbiosis is two organisms living intimately together, and this chapter examines the physiological basis of the interaction. A mycorrhiza is comprised of two distinctly different organisms, a plant and a fungus, that interface down to the molecular level. Because of this intimate physical closeness, the biochemistry, physiology, and ecology become highly intertwined. At the most basic definitional level, the fungus picks up nutrients and water in the soil, transfers those resources to the host, in exchange for carbon fixed by the plant from the atmosphere. This physical dimension means that resources available to one partner are less available to the other. But both sets of resources are essential to both organisms.