Nisin is a 34-amino acid peptide known as a bacteriocin. It belongs to class-Ia bacteriocins known as lantibiotics. It is the most characterized and commercially used antimicrobial peptide. It was commercialized in the 1950s for the prevention of late blowing in cheese by Clostridium. Nisin is heat-stable and a cationic, low-molecular-weight peptide produced by the bacterial dairy starter Lactococcus lactis subsp. lactis. The growth of a wide spectrum of Gram-positive bacteria, including the pathogens Listeria and Staphylococcus aureus, and spore-forming bacteria such as Bacillus and Clostridium can be prevented by nisin. Subjects addressed in the chapter include nisin structure and physicochemical properties, antimicrobial spectrum, factors affecting the antimicrobial activity (food composition, temperature, pH, proteolytic enzymes, food processes, other preservatives, bacterial load), application as a preservative to dairy products, meat products, seafood, fruits and vegetables, beverages, and other food products, use of nisin in food packaging systems, and regulatory status.