This dissertation explores two diametrically opposed behavioral risk factors. The first risk factor, television watching, is the most prevalent leisure activity in the United States. The second, physical activity, is one of the most important modifiable risk factors for chronic disease. The first aim was to examine the association of television watching with physical activity and diet in the Atherosclerosis Risk in Communities Study. Television exposure, physical activity, and dietary information were collected via self-report at baseline and six years later in 12,678 men and women ages 45 to 64 years at baseline. Participants who reported high television exposure were more likely to be inactive and have a poor dietary profile. These results persisted over the six years of follow-up, regardless of modeling strategy. Risks associated with sedentary behaviors, such as television watching, have been neglected in public health research. This work highlights the need for further research on measurement, determinants, and risks of sedentary behaviors. A better understanding of behavioral risk factors such as physical activity requires continued research on current questionnaires and measurement tools used to quantify these behaviors. The test-retest reliability of the Women's Health Initiative physical activity questionnaire was examined in 1092 women as part of the second aim. Mild physical activity had lower test-retest reliability than moderate, vigorous, and walking physical activity, which were moderately to substantially reliable. Walking is the most common leisure physical activity engaged in by adult women. The Women's Health Initiative, with its reliable and unique questionnaire offered an opportunity to examine the independent effects of walking intensity, frequency, and duration on risk of coronary heart disease. After a decade of follow-up, baseline intensity, frequency, and duration of walking were all associated with lower risk of coronary heart disease in 71, 502 women from 40 centers across the United States. The highest category of walking intensity was associated with a 60% reduction in risk relative to the lowest category. Weaker associations were observed for frequency and duration of walking. To untangle the effect of each walking component (i.e., intensity, frequency, and duration) from their contribution to energy expenditure, a control variable estimating recreational physical activity energy expenditure was introduced in the models. The addition of this control variable did not appreciably change the results. Strengths and limitations of this approach are highlighted. This dissertation highlights several areas for future research in physical activity epidemiology. The first is a pressing need to better understand the impact of sedentary behaviors on health. The second is the need to develop better measurement tools for research and surveillance. Lastly, new epidemiologic methods need to be applied to help understand the specific health benefits from different types of physical activities or components of activity, such as frequency, intensity, and duration
