Listeria refers to a group of bacteria of which one species, Listeria monocytogenes, is predominant in causing a serious and potentially fatal disease known as listeriosis. The bacterium poses particular problems to both the food industry and regulator because it is widespread, can grow in chilled foods, and is relatively resistant to control measures when compared with other non-spore-forming foodborne pathogens. Characteristically, it infects the young, old, pregnant, and immunocompromised and generally causes its most significant clinical outcomes (e.g., septicemia, meningitis, fetal death) in these populations.