Nearly two thirds of American adults are overweight or obese, placing them at increased risk of obesity-related chronic diseases, including cancer. Although it has been known for quite some time that obesity increases initial risk of breast, colon, and endometrial cancer, new data implicate obesity as a risk factor for ovarian and thyroid cancers. In addition, recent evidence shows that obesity is associated with poor survival from most cancers. Obesity influences several biological pathways, including those related to glucose homeostasis, insulin-like growth factor signaling, and inflammation. Dysregulation of these pathways in obesity may provide the biological link between obesity and carcinogenesis.