There are many times when our cells need to move. Mobile cells guide our body's formation (embryonic development). Immune cells roam to capture unwanted intruders. And healing cells (fibroblasts) migrate to mend wounds. But not all movement is desirable: Tumors are most dangerous when cancer cells gain the ability to travel throughout the body (metastasis). Certain bacteria and viruses can harness the cells' motility machinery to invade our bodies. Understanding how cells move—and the rod-like actin filaments that drive the process—is key to learning how to halt or promote motility to improve human health.