Listeria monocytogenes and other bacterial pathogens move in the host cell cytoplasm, propelled by continuous actin assembly at one pole of the bacterium. This actin-based motility requires the presence of the bacterial proteins ActA on L. monocytogenes and IcsA on Shigella flexneri. There have been several major discoveries in the past year: the discovery of the polar distribution of ActA and IcsA on the bacterial surface; the demonstration that bacterial ActA is phosphorylated in infected host cells; the involvement of some host cell proteins, particularly profilin; and the dramatic effect of ActA expression in transfected eukaryotic cells. A cell-free system that reconstitutes faithfully the actin-based motility of L. monocytogenes promises to be instrumental in the further dissection of this fascinating phenomenon.