Amino acids and small peptides are very efficiently reabsorbed at the level of the kidney proximal tubule by a series of specialized membrane proteins. In a first step, they are taken up into tubular epithelial cells via secondary active transporters localized at the luminal brush border membrane. Most of these amino acids as well as the amino acids generated by the intracellular hydrolysis of the di-/tripeptides are then transported out of the tubular cell across the basolateral membrane into the interstitial space. This second transport step is mediated by another set of transporters that function as facilitated diffusion pathways and obligatory exchangers. Besides reabsorbing amino acids, proximal tubule cells also contribute to body amino acid metabolism, for instance by producing L-arginine, L-tyrosine and L-serine or by metabolizing L-glutamine to produce ammonia for the urinary excretion of protons. Mutations of genes encoding luminal or basolateral amino acids transporters of proximal tubule cells can cause aminoacidurias and lead in some cases, for instance of cystinuria or lysinuric protein intolerance (LPI), to severe diseases.