The renal anatomy, physiology, and cellular mechanisms involved in tubular transport of organic acids (anions) and bases (cations) are reviewed. Drugs that are renally secreted are prone to significant and complex drug-drug interactions, and knowledge of the underlying mechanisms is important. Several clinical studies involving commonly used cationic drugs (e.g., cimetidine, trimethoprim, and procainamide) are cited as examples of drug interactions involving renal transport mechanisms.