Listeria monocytogenes emerged as an important foodborne pathogen in the latter part of the 20th century. Clinical syndromes caused by this microorganism include sepsis in the immunocompromised patient, meningoencephalitis in infants and adults, and febrile gastroenteritis. Focal infections at other sites are less frequent. Listeria species are commonly found in raw and unprocessed food products. Major outbreaks of listeriosis, with high morbidity and mortality, have been caused by a variety of foods, including soft cheeses, delicatessen meats, and vegetable products. Improved detection methods, dietary recommendations, and, in some cases, preemptive antibiotic treatment or prophylaxis have reduced the incidence of sporadic listeriosis infections in the United States. Microbial virulence factors distinguishing environmental strains of L. monocytogenes from invasive strains causing foodborne illness and host factors promoting human infection remain incompletely understood.