Spatially controlled polymerization of the cytoskeleton protein actin generates the motile force required for biological processes such as the formation of cellular protrusions like lamellipodia, cancer cell metastases, and the intracellular movements of bacterial and viral pathogens within the cytosol of infected cells. In the May 5 PNAS, Paula Giardini and colleagues at Stanford University School of Medicine describe a model system enabling a qualitative and quantitative analysis of the mechanism of force generation in this poorly understood form of biological motility (PNAS, DOI:10.1073/pnas.1031670100, May 5, 2003).