We hypothesize that a population based approach that emphasizes physical activity maintenance can substantially increase physical activity levels in a defined populations of older adults. This hypothesis is tested in a 24-month randomized trial evaluating an innovative, theory-based behavioral intervention to maintain physical activity in a random sample of 50-70 year old adults who have recently become at least moderately active. One thousand (1000) subjects will be randomized to one of two experimental groups: 1) a "usual care" control group, and 2) an interactive phone- and mail-based intervention program tailored to maintaining physical activity in older adults. The primary outcome measures are: 1) physical activity, assessed as kcals/wk expenditure; and 2) physical activity maintenance, assessed as follow-up kcals/wk expenditure relative to baseline. Careful measurement of the penetration of the intervention into a well characterized older adult population, and the costs of the intervention, will be assessed. Psychosocial and behavioral mediators of physical activity maintenance will also be examined.