Current understanding of the features and mechanisms of renal acidification along the nephron segments is summarized. Emphasis is placed on recent approaches to the "structure and function" of H/HCO3 transport proteins by molecular techniques. The amount of renal acidification is strictly matched to the endogenous production of non-volantile acids. This regulation is handled by many factors, including blood acid-base parameters, aldosteron, distal delivery of Na and accompanying anions, extracellular fluid volume, and PTH. Cellular mechanisms of these regulatory factors are summarized.