Obesity is a problem of epidemic proportions in many developed nations. Increased body mass index and obesity are associated with a significantly worse outcome for many cancers. Breast cancer risk in the postmenopausal setting and poor disease outcome for all patients is significantly augmented in overweight and obese individuals. The expansion of fat tissue involves a complex interaction of endocrine factors known as adipokines and cytokines. High cytokine levels in primary breast cancers and in the circulation of affected patients have been associated with poor outcome. This review summarizes the how cytokine production in obese adipose tissue creates a chronic inflammatory microenvironment that favors tumor cell motility, invasion, and epithelial-mesenchymal transition to enhance the metastatic potential of tumor cells. Many of the cytokines associated with a proinflammatory state are not only upregulated in obese adipose tissue but may also stimulate the self-renewal of cancer stem cells. Thus, enhanced cytokine production in obese adipose tissue may serve both as a chemoattractant for invading cancers and to augment their malignant potential. These new mechanistic insights suggest that the current obesity epidemic will presage a significant increase in cancer incidence, morbidity, and mortality in the next few decades.