Summary: Nitric oxide synthase (NOS) is a critical enzyme for the production of the messenger molecule nitric oxide (NO) from L-arginine. NOS enzymes require tetrahydrobiopterin as a cofactor for NO synthesis. Besides being one of the few enzymes to use this cofactor, the role of tetrahydrobiopterin in NOS catalytic mechanism is different from other enzymes: during the catalytic cycle of NOS, tetrahydrobiopterin forms a radical species that is again reduced, thus effectively regenerating after each NO synthesis cycle. In this review, we summarize our current knowledge about the role of tetrahydrobiopterin in the structure, function, and catalytic mechanism of NOS enzymes. © 2013 IUBMB Life 65(4):358–365, 2013.