Bacteria that infect the body via mucosal surfaces typically display one or more of the following characteristics: (i) colonization and penetration of mucosal epithelia; (ii) evasion of host defences; (iii) multiplication in vivo, and (iv) production of tissue damage. The Gram-negative enteric pathogens, Yersinia enterocolitica and Y. pseudotuberculosis, display all of these to some extent and thus serve as excellent models for the investigation and understanding of bacterial pathogenesis. Several virulence determinants of Y. enterocolitica and Y. pseudotuberculosis have been identified and characterized. These factors act in concert at various stages of the infection to permit bacterial survival and replication in the myriad of environments present in host tissues, including the lumen of the gastrointestinal tract, and the intra- and extracellular compartments of reticulo-endothelial tissues. Expression of many of these virulence determinants is genetically regulated ensuring that they are produced when the bacteria require them. Together, they contribute to the remarkable versatility of Y. enterocolitica and Y. pseudotuberculosis as pathogens. Several of the factors involved in virulence have homologues in other human intestinal pathogens, including Salmonella and Shigella species and diarrhoea-associated strains of Escherichia coli, thus emphasizing the relevance of research into yersiniosis as a means of furthering our understanding of bacterial virulence in general, while providing tantalizing clues to the origin and evolution of bacterial pathogens.