 The WHISH trial applies state-of-the science behavioral principles and currently available technologies to deliver a physical activity intervention without face-to-face contact to 25,000 older U.S. women expected to consent. It includes an algorithm-driven interactive voice response IVR system, built to complement the National Institute of Aging NIA Go4Life Exercise Physical Activity materials 3, which provides tailored advice on how to achieve nationally recommended levels of PA and overcome barriers to exercise, with a means for self-monitoring and setting personal goals. The intervention builds upon evidence-based behavioral science principles and intervention components that have proven to be effective in increasing PA in older women, with innovative adaptive approaches to tailoring the delivery to meet individual personal needs. 