Breast cancer is a malignant tumor that starts in the cells of the breast. It is mostly found in women. The cells grow abnormally and in an uncontrolled way. Breast cancer is considered one of the main health issues over the world with about one million cases diagnosed each year. Obesity is associated with increased risk of breast cancer. However, the complexity of the underlying mechanisms, together with the interplay of diet and physical activity and the role of adipose tissue, pose challenges to our understanding of the basis of this increased risk. Obesity increases breast cancer risks through different mechanisms such as increased levels of estrogens due to excessive aromatization activity of the adipose tissue, overexpression of pro-inflammatory cytokines, insulin resistance, hyperactivation of insulin-like growth factors (IGFs) pathways, adipocyte-derived adipokines, hypercholesterolemia and excessive oxidative stress contribute to the development of breast cancer in obese women.