Nitric oxide synthases (NOSs) generate nitric oxide (NO) and the by-product l-citrulline, via the catalytic combination of l-arginine and molecular oxygen. In mammals, there are three NOS genes: nNOS (NOS1), iNOS (NOS2) and eNOS (NOS3). The molecular structure, enzymology and pharmacology of these enzymes have been well defined, and reveal critical roles for the NOS system in a variety of important processes. The studies of NOS enzymes using knockout and transgenic mouse models have provided an invaluable contribution, highlighting critical roles in neuronal, renal, pulmonary, gastro-intestinal, skeletal muscle, reproductive and cardiovascular biology. This review will outline the data gleaned from complementary knockout and transgenic over-expression models in mice, and focus on the interactions between NOS enzymes and pathophysiology of the vascular system. These studies are a paradigm for the near future, which will involve the translation of an enormous amount of genomic data into physiological insights that penetrate the realms of both health care and biology.