In addition to the spectrum of conditions known collectively as the Metabolic Syndrome, obesity is now recognized to be associated with increased risk of several cancers including colon, endometrial, and breast cancer. Obesity and carcinogenesis share 2 characteristics in common. On the one hand, they involve inflammatory pathways, and on the other hand, they involve dysregulated metabolism. In this review we focus on postmenopausal breast cancer and discuss the metabolic and cellular mechanisms whereby obesity and breast cancer are related. Because a majority of postmenopausal breast tumors are estrogen responsive, we include a discussion of the action of obesity-related factors on estrogen formation within the breast.