Obesity is the largest pandemic, the human being has undergone. Obesity has been linked to several health-related problems, but recent epidemiological studies have uncovered a link between excess body weight and certain types of cancers. We have begun to realize the complexity of the biological mechanisms that link obesity and being overweight to many forms of cancer. In this chapter, I briefly review these epidemiological findings and the three molecular pathways currently proposed to link body weight gain and cancer risk. Despite the fact that these molecular pathways, vary and depend on both sex and the type of cancer, I briefly discuss the possible roles of three hormonal systems, the adipose derived hormones, the Insulin-Like Growth Factor (IGF) axis and sex steroids, in linking obesity and cancer risk.