Disorders of amino acid transport have been very influential in the elucidation of amino acid absorption in the kidney and intestine. Molecular identification of the transport systems mutated in these disorders highlights the complexity of amino acid transport and demonstrates its links to more complex pathologies such as blood pressure regulation and epithelial cell differentiation. Hartnup disorder is an autosomal recessive disorder. It is caused by mutations in the general neutral amino acid transporter SLC6A19. The transporter requires one of the two auxiliary proteins collectrin or ACE2 for surface expression. Collectrin is predominantly found in the kidney, whereas ACE2 is found predominantly in the intestine. Iminoglycinuria (IG) was first described fifty years ago as an autosomal recessive abnormality of renal transport of glycine and of the imino acids, proline and hydroxyproline. The disorder is caused by mutations in the common proline and glycine transporter PAT2 (SLC36A2). Hyperglycinuria (HG) is frequently observed in heterozygotes. In some individuals SLC36A2 mutants retained residual transport activity; in those cases the urinary phenotypes were modified by additional mutations in the proline transporter IMINO (SLC6A20).