Proximal tubular reabsorption of inorganic phosphate (P(i) is a major determinant of the extracellular concentration of phosphate. The net proximal reabsorption of phosphate largely depends on the rate of apical sodium-dependent transport of P(i) (Na/P(i) cotransport), which is controlled by various hormonal and nonhormonal factors. Renal Na/P(i) cotransport systems that represent proximal apical Na/P(i) cotransport systems have recently been identified. Molecular knowledge of proximal tubular Na/P(i) cotransport will lead to a new understanding of the cellular mechanisms of the physiologic control of proximal P(i) reabsorption and to elucidation of the pathophysiologic mechanisms impairing P(i) homeostasis.