Nitric Oxide (NO) is a gaseous free radical with a short half-life in vivo of a few seconds or less. It is a pleiotropic biological mediator that regulates diverse activities ranging from neuronal function to immune system regulation. NO is catalyzed by enzymes of the Nitric Oxide Synthase (NOS) family, which include neuronal NOS (nNOS/NOS1), inducible NOS (iNOS/NOS2), and endothelial NOS (eNOS/NOS3). NO is lipid soluble, so it is not stored by synthesized de novo and freely diffuses across lipid membranes. NO mediates its effects on target cells via several different mechanisms. For instance, NO can activate Guanylyl Cyclase, which catalyzes the formation of the second messenger cGMP. cGMP is implicated with a range of biological functions such as regulating smooth muscle contractility, cell survival, proliferation, axon guidance, synaptic plasticity, inflammation, angiogenesis, and the activity of cyclic nucleotide-gated channels. NO also functions as an anti-tumor and anti-microbial agent via mechanisms that include its conversion to peroxynitrite, the formation of S-nitrosothiols, and the depletion of arginine. Additionally, NO suppresses mitochondrial respiration through the inhibition of Cytochrome Oxidase, and may modify protein activity through post-translational nitrosylation. Altered levels of NO have been shown to be associated with sepsis, reproduction, infection, hypertension, exercise, type 2 diabetes, hypoxia, and cancer.