Tryptophan hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin (5-HT). Increases or decreases in TPH catalytic activity can therefore lead to corresponding changes in the neuronal content of 5-HT. TPH requires a reduced pteridine cofactor, molecular oxygen, and nonheme iron to hydroxylate its substrate l-tryptophan. Alterations in the tissue content of any of these four factors could change TPH activity. Posttranslational modifications of TPH, including phosphorylation and cysteine oxidation, cause significant changes in its catalytic activity. The discovery of a brain-specific isoform of TPH, termed TPH2 to distinguish this enzyme from the peripheral form TPH1, opened new avenues of research that allowed in-depth studies of TPH molecular function, solutions of 3D crystal structures, creation of TPH1 and TPH2 knockout mouse lines, and the successful search for specific inhibitors of TPH isoforms some of which are entering clinical trials for the treatment of disorders linked to excess peripheral 5-HT.