Listeriosis is a severe human and animal disease caused by two species of pathogenic bacteria from the genus Listeria, L. monocytogenes and L. ivanovii. In humans, listeriosis is overwhelmingly a foodborne disease, yet much remains to be learned regarding the transmission dynamics of pathogenic Listeria from the environment, through food, to humans. Similarly, our understanding of the various host, pathogen and environmental factors that impact the pathogenesis of listeriosis at the cellular and molecular level is incomplete. This review will summarize what is currently known about animal and human listeriosis, detail the pathogen, host and environmental factors that contribute to pathogenesis and, finally, examine the interactions among those factors that influence the occurrence of human infection.