In this article, the authors examine how temperature and precipitation affect the probability that a retired American between the ages of 65 and 90 walks at least 2.5 hr/wk, using longitudinal data on walking frequency from the Consumption and Activities Mail Survey, a subpanel in the Health and Retirement Survey. Walking behavior is linked with monthly temperature and precipitation data from weather-station reports. The authors found that higher temperatures were associated with a higher probability of walking at least 2.5 hr/wk for women. In contrast, higher temperatures are associated with a lower probability of walking at least 2.5 hr/wk among men. Precipitation is not significantly associated with walking behavior for either gender.