A 12-month randomized trial in which women and men ages 50 years and above living in the San Francisco Bay area who meet medical and physical activity criteria, following a baseline evaluation, be randomized to one of three study arms: telephone-supervised, home-based physical activity instruction via a trained health educator; telephone-supervised, home-based physical activity instruction via a supervised peer facilitator; or an attention-control arm consisting of health educator-based telephone delivery of non-physical activity oriented health information and support. All subjects will have a follow-up evaluation at 6 and 12 months.