Bacteria of Shigella spp. (S. boydii, S. dysenteriae, S. flexneri and S. sonnei) and enteroinvasive Escherichia coli (EIEC) are responsible for shigellosis in humans, a disease characterized by the destruction of the colonic mucosa that is induced upon bacterial invasion. Shigella spp. and EIEC strains contain a virulence plasmid of approximately 220 kb that encodes determinants for entry into epithelial cells and dissemination from cell to cell. This review presents the current model on mechanisms of invasion of the colonic epithelium by these bacteria and focuses on their pathogenicity factors, particularly the virulence plasmid-encoded type III secretion system.