Almost all cancer cells have overcome the normal cellular signals that prevent continued division, and thus are immortal. Normal cells from young individuals can divide many times, but they have not accumulated all the other changes needed to make a cell malignant. In most instances, cells become senescent (aged) before they become malignant. Cellular aging and cancer appear to be linked, and the mechanisms regulating aging may act as a cancer brake. A major challenge is to find out how to make cancer cells mortal to inhibit their growth, and how to increase the lifespan of healthy cells.