Enteropathogenic Escherichia coli (EPEC) are a major cause of paediatric diarrhoea and a model for the family of attaching and effacing (A/E) pathogens. Enteropathogenic Escherichia coli encode a type III secretion system (TTSS) to transfer effector proteins into host cells, a process which is essential for virulence. In addition to generation of A/E lesions, the TTSS is also implicated in the ability of EPEC to invade cultured cells but the effector proteins responsible for promoting invasion have not been identified. In this paper we confirm the requirement of TTSS in EPEC invasion and demonstrate important roles for the Map and Tir effector molecules. Whereas in trans expression of Tir in the tir mutant restored invasion to wild-type levels, similar complementation of the map mutation by in trans expression of Map results in a hyperinvasive phenotype. The Map effector protein has two distinct functions within host cells, mediating Cdc42-dependent filopodia formation and targeting mitochondria to elicit dysfunction. The former function appears to be related to Map's ability to promote invasion as this was inhibited by interference with Cdc42 signalling. Conversely, Map targeting to mitochondria is not necessary for invasion. Promotion of EPEC invasion by Tir appears to involve interaction with intimin but is independent of pedestal formation, and intimin-Tir interaction is neither necessary nor sufficient for invasion. Comparison of the invasiveness of strains lacking Tir and/or Map with wild-type or mutant strains expressing the effectors in trans provides evidence that Map and Tir stimulate invasion by synergistic mechanisms. This synergism, which is in stark contrast to the antagonistic actions of Map and Tir in regulating filopodia and pedestal formation, further illustrates the complex interplay between EPEC effectors.