Context: Current public health campaigns to reduce obesity and type 2 diabetes have largely focused on increasing exercise, but have paid little attention to the reduction of sedentary behaviors. Objective: To examine the relationship between various sedentary behaviors, especially prolonged television (TV) watching, and risk of obesity and type 2 diabetes in women. Design, Setting, and Participants: Prospective cohort study conducted from 1992 to 1998 among women from 11 states in the Nurses' Health Study. The obesity analysis included 50277 women who had a body mass index (BMI) of less than 30 and were free from diagnosed cardiovascular disease, diabetes, or cancer and completed questions on physical activity and sedentary behaviors at baseline. The diabetes analysis included 68497 women who at baseline were free from diagnosed diabetes mellitus, cardiovascular disease, or cancer. Main Outcome Measures: Onset of obesity and type 2 diabetes mellitus. Results: During 6 years of follow-up, 3757 (7.5%) of 50277 women who had a BMI of less than 30 in 1992 became obese (BMI ≥30). Overall, we documented 1515 new cases of type 2 diabetes. Time spent watching TV was positively associated with risk of obesity and type 2 diabetes. In the multivariate analyses adjusting for age, smoking, exercise levels, dietary factors, and other covariates, each 2-h/d increment in TV watching was associated with a 23% (95% confidence interval [CI], 17%-30%) increase in obesity and a 14% (95% CI, 5%-23%) increase in risk of diabetes; each 2-h/d increment in sitting at work was associated with a 5% (95% CI, 0%-10%) increase in obesity and a 7% (95% CI, 0%-16%) increase in diabetes. In contrast, standing or walking around at home (2 h/d) was associated with a 9% (95% CI, 6%-12%) reduction in obesity and a 12% (95% CI, 7%-16%) reduction in diabetes. Each 1 hour per day of brisk walking was associated with a 24% (95% CI, 19%-29%) reduction in obesity and a 34% (95% CI, 27%-41%) reduction in diabetes. We estimated that in our cohort, 30% (95% CI, 24%-36%) of new cases of obesity and 43% (95% CI, 32%-52%) of new cases of diabetes could be prevented by adopting a relatively active lifestyle (<10 h/wk of TV watching and ≥30 min/d of brisk walking). Conclusions: Independent of exercise levels, sedentary behaviors, especially TV watching, were associated with significantly elevated risk of obesity and type 2 diabetes, whereas even light to moderate activity was associated with substantially lower risk. This study emphasizes the importance of reducing prolonged TV watching and other sedentary behaviors for preventing obesity and diabetes.