Enteropathogenic and enterohaemorrhagic Escherichia coli are closely related enteric pathogens whose ability to cause disease in humans is linked with a capacity to deliver bacterial 'effector' proteins into host epithelia to alter cellular physiology. Although the essential role of the locus of enterocyte effacement (LEE) pathogenicity island, which encodes effector proteins and the delivery machinery, has been established, more recent studies are uncovering additional layers of complexity. This is illustrated by the emerging multifunctional nature of the effectors and their ability to work together in redundant, synergistic and antagonistic relationships. Furthermore, new virulence-associated factors are continually being uncovered that are encoded outside the LEE pathogenicity island, some of which are not injected into host cells.