This article reviews the existing epidemiological evidence on the association between physical activity and cancer. Relatively little is known about the role that increased physical activity may have in protecting humans from malignancy. Herein, studies are reviewed on the basis of the type of physical activity exposure studied: occupational or leisure time. Results suggest that if any consistencies emerge, there may be a protective effect of increased physical activity exposure upon cancer of the colon, and no effect on cancer of the rectum, in men. Increased physical activity in women during college years may protect against certain reproductive system cancers. Methodological deficiencies in assessment of physical activity (total exposure over lifetime) and in lack of control for other potential explanations block further, more solid conclusions. The fact that an association has been demonstrated in spite of these drawbacks, however, indicates the importance of further study. Suggested directions for future work are presented.