There is growing scientific evidence linking excess body weight to breast cancer risk. However, there is nocommon consensus on this relation due partly to methodologies used, populations studied and the cancer subtype.We report here a summary of the present state of knowledge on the role of overweight and obesity in pathogenesisof breast cancer and possible mechanisms through which excess body weight might influence the risk, focusingon the role of oxidative stress in breast cancer etiology. The findings demonstrate duality of excess body weightaction in dependence on menopausal status: a statistically significant increased risk in postmenopausal overweight/obese women and non-significant preventive effect among premenopausal women. Due to several gaps in theliterature on this topic, additional studies are needed. Future research should address factors influencing theexcess body weight - breast cancer relationship, such as race/ethnicity, tumor subtype, receptor status, the mostappropriate measure of adiposity, reproductive characteristics, and lifestyle components.