 20% of cancer cases are estimated to be caused by being obese, with a stronger association found for certain types such as endometrial, esophageal adenocarcinoma, colorectal, postmenopausal breast, prostate, and renal. The underlying processes linking cancer to obesity include insulin, insulin-like growth factor I, sex steroids, and adipokines, with potential mechanisms such as chronic inflammation, oxidative stress, cross-talk between tumor cells and surrounding adipocytes, migrating adipostromal cells, obesity-induced hypoxia, shared genetic susceptibility, and functional defeat of immune function. This article was authored by Giovanni D'Apergola and Franco Sylvesteris.