Adiposity is a well-recognized risk factor of type 2 diabetes and cardiovascular disease, and recently there is increasing evidence that excess body weight is an avoidable cause of cancer, including gastrointestinal, endometrial, esophageal adenocarcinoma, colorectal, postmenopausal breast, prostate, and renal malignancies. The mechanisms whereby adiposity is associated with tumor development remains not well understood. There are some most studied hypothesized mechanisms such as, high levels of insulin and free levels of insulin-like growth factors, sex hormones, adipocytokines, and inflammatory cytokines, adiposity-induced hypoxia, and so on. The potential mechanisms and conclusions in adiposity associated with increased risk for developing malignancy, and the underlying cellular and molecular mechanisms will be studied very well in the near future.