Nitric oxide (NO) is a ubiquitous messenger and effector molecule that plays important roles in regulating a perplexing number of different cell functions in prokaryotes and across the plant and animal kingdom. In mammalian cells, NO is formed either via oxidation of the amino acid, L-arginine by enzymes of the NO-synthase family or via enzymatic/non-enzymatic reduction of nitrite. While the former pathway is dependent on the availability of oxygen, much of the latter is inhibited by its presence. Together, these processes may serve the function of providing sufficient amounts of NO along the entire physiological oxygen gradient, but for exactly what purpose is not entirely clear. Depending on the tissue under investigation, the amounts of NO that can be produced from nitrite under hypoxic conditions appear to be rather high, and the level of redundancy of enzyme systems capable of reducing nitrite to NO as well as their subcellular localization are puzzling. Similarly unclear is what factors govern production and stability of nitrosation and nitrosylation products (e.g., S-nitrosothiols and NO-heme complexes) in health and disease and what defines “nitrosative stress”, which roles NO, nitrite and nitrate play during sustained periods of hypoxia (conditions under which vasodilatation would seem unlikely to provide much benefit), and how inflammation and nutritional status affect these pathways. This presentation will focus on the occurrence and mechanisms of formation of S- and N-nitrosation (nitrosothiols and nitrosamines) and heme nitrosylation (NO-heme) products in blood and tissues under a number of different conditions in vivo. Particular emphasis will be placed on their usefulness as biomarkers of disease in translational studies and misconceptions as well as open questions that continue to hamper progress in this area of research.