 1. Myeloperoxidase, MPO, is a heme-containing peroxidase produced mostly from polymorphonuclear neutrophils, with the active enzyme being a homodomer of heavy and light-chain protomers. 2. MPO uses H2O2 to oxidize several halides and pseudohalides to form different hypohalous acids, which are involved in its antibacterial activities through the production of reactive oxygen and reactive nitrogen species. 3. Controlled MPO release at the site of infection is important for its efficient activities, but uncontrolled degranulation can lead to tissue damage even in absence of inflammation and has been linked with several types of tissue injuries and chronic diseases such as rheumatoid arthritis, cardiovascular diseases, liver diseases, diabetes, and cancer. This article was authored by Amjade Khan, Muhammad A. Alsali, and Arshad H. Ramani.