A key process in overall P<sub>i</sub>-homeostasis is renal proximal tubular reabsorption of inorganic phosphate (P<sub>i</sub>), which involves secondary active sodium/phosphate (Na<sup>+</sup>/P<sub>i</sub>) cotransport reabsorption at the brush border membrane. Among the two different molecularly identified Na<sup>+</sup>/P<sub>i</sub> cotransporters, the type-IIa Na<sup>+</sup>/P<sub>i</sub> cotransporter (NaPi-IIa) accounts for up to 70% of brush border membrane transport. Regulation of renal P<sub>i</sub> reabsorption centers around brush border membrane insertion and retrieval of transporter protein under the influence of hormonal and nonhormonal factors. Immunohistochemical and fluorescence techniques have provided new insights into the tissue distribution and the regulation processes. The intrinsic electrogenicity of NaPi-IIa, has allowed detailed studies of the transport kinetics of NaPi-IIa and, combined with mutagenesis methods, structure-function information at the protein level is emerging.