Tetrahydrobiopterin-dependent systems occupy several interesting niches in biological chemistry, as metalloproteins that activate O2 for highly specific oxidations, as regulators for aromatic amino acid metabolism, and as mediators for the health and proper functioning of the brain. Phenylalanine, tyrosine, and tryptophan hydroxylases perform critical and potentially rate-limiting steps in phenylalanine catabolism (PAH), epinephrine/catecholamine biosynthesis (TyrH), and serotonin biosynthesis (TrpH). This review is intended to be comprehensive with regard to the biophysical, structural, and catalytic properties of these three enzymes as reported to the end of 1995. One cannot discuss mechanistic aspects of these non-heme iron-dependent, pterindependent amino acid hydroxylases without an extensive consideration of the coordination chemistry of iron, the organic chemistry of pteridines, and the reactivity of each, which are duly emphasized in the current review. For the more frequently reviewed aspects of these enzymes, including the extensive work on PAH mutations and population genetics1 and medical aspects including neurological effects, pharmacological studies, and developmental regulation, 2, 3