Myeloperoxidase (MPO) is a heme-containing enzyme belonging to the XPO subfamily of peroxidases. It is an abundant neutrophil and monocyte glycoprotein that catalyzes the hydrogen peroxide dependent formation of hypochlorus acid (HOCl) and other reactive species. Enzymatically active MPO is a disulfide-linked tetramer that contains two heme groups and two copies each of the heavy and light chains. MPO binds Albumin, MMR, Cytokeratin 1 on vascular endothelial cells, HMW Kininogen, and Integrin CD11b/CD18 on neutrophils. These interactions promote MPO clearance, a reduction of nitric oxide and bradykinin levels, reduced vasodilation, and continued neutrophil activation. Neutrophil MPO is stored in cytoplasmic azurophilic granules. Upon cellular activation and degranulation, MPO is delivered into phagosomes where it is required for the killing of phagocytosed bacteria. Activated neutrophils also release granule contents extracellularly. Elevated plasma MPO levels have been associated with a variety of clinical conditions including systemic inflammation, eclampsia, risk of cardiovascular events, vascular endothelial dysfunction, severity of multiple sclerosis, and prospective mortality and oxidative stress during hemodialysis.