Nitric oxide synthase (NOS) catalyzes the oxidation of one l-arginine guanidinium N atom to nitric oxide (NO). NOS consists of a heme domain linked to a flavin mononucleotide (FMN)/flavin adenine dinucleotide (FAD) reductase that shuttles electrons from nicotinamide adenine dinucleotide phosphate (NADPH) to the heme. This review summarizes various aspects of NOS structure and function derived from crystal structures coupled with a wealth of biochemical and biophysical data. This includes the binding of diatomic ligands, especially the product, NO, whose binding to the heme iron blocks enzyme activity. An unusual feature of NOS catalysis is the strict requirement for the essential cofactor, tetrahydrobiopterin (H4B). It now is generally agreed that H4B serves as an electron donor to the heme-oxy complex. The reason NOS may have recruited H4B as an electron transfer cofactor is to provide rapid coupled proton/electron transfer required for O2 activation. NOS is a highly regulated enzyme which is controlled by calmodulin (CaM) at the level of electron transfer within the FMN/FAD reductase and between the reductase and heme domains. Recent crystal structures provide a basis for developing models on the structural underpinnings of NOS regulation. In addition to the complex and fascinating functional and regulatory features of NOS, NOS is an important therapeutic target. Crystal structures have revealed the structural basis of isoform-selective inhibition by a group of dipeptide inhibitors which opens the way for structure-based inhibitor design.