Abstract: Scientific evidence for the primary prevention of cancer caused by physical activity of regularmoderate-intensity or greater is rapidly accumulating in this field. About 300 epidemiologic studies on theassociation between physical activity and cancer risk have been conducted worldwide. The objectives of this paperwere three-fold: (i) to describe briefly the components of physical activity and its quantification; (ii) to summarizethe most important conclusions available from comprehensive reports, and reviews of the epidemiologicindividual and intervention studies on a role physical activity in cancer prevention; (iii) to present proposedbiological mechanisms accounting for effects of activity on cancer risk. The evidence of causal linked physicalactivity and cancer risk is found to be strong for colon cancer - convincing; weaker for postmenopausal breastand endometrium cancers - probable; and limited suggestive for premenopausal breast, lung, prostate, ovary,gastric and pancreatic cancers. The average risk reductions were reported to be 20-30%. The protective effectsof physical activity on cancer risk are hypothesized to be through multiple interrelated pathways: decrease inadiposity, decrease in sexual and metabolic hormones, changes in biomarkers and insulin resistance, improvementof immune function, and reduction of inflammation. As there are several gaps in the literature for associationsbetween activity and cancer risk, additional studies are needed. Future research should include studies dealingwith limitations in precise estimates of physical activity and of a lack of consensus on what defines sedentarybehavior of individuals and those linked with the proposed biomarkers to cancer risk and controlled exerciseintervention trials.