Nitric oxide (NO), a key regulator of cardiovascular function, is synthesized from L-arginine and oxygen by the enzyme nitric oxide synthase (NOS). This reaction requires tetrahydrobiopterin (BH4) as a cofactor. BH4 is synthesized from guanosine triphosphate (GTP) by GTP cyclohydrolase I (GTPCH) and recycled from 7,8-dihydrobiopterin (BH2) by dihydrofolate reductase. Under conditions of low BH4 bioavailability relative to NOS or BH2, oxygen activation is “uncoupled” from L-arginine oxidation, and NOS produces superoxide (O2−) instead of NO. NOS-derived superoxide reacts with NO to produce peroxynitrite (ONOO−), a highly reactive anion that rapidly oxidizes BH4 and propagates NOS uncoupling. BH4 depletion and NOS uncoupling contribute to overload-induced heart failure, hypertension, ischemia/reperfusion injury, and atrial fibrillation. L-arginine depletion, methylarginine accumulation, and S-glutathionylation of NOS also promote uncoupling. Recoupling NOS is a promising approach to treating myocardial and vascular dysfunction associated with heart failure.