Numerous epidemiological cohort and case-control studies showed that type 2 diabetes is a risk factor for cancer and that metformin therapy is associated with a significant reduction in the incidence of cancer and cancer-related death when compared to other glucose-lowering agents. Such beneficial effect is observed whatever the type of cancer, but seems to be more prominent in case of gastrointestinal and breast cancers. In general, the protective effect was more evident in observational cohort studies (however, more exposed to bias due to confounding factors) than in case-control studies. However, the results of the rather rare controlled clinical trials available are not conclusive, but none of them was performed with the objective to specifically assess cancer risk. Several meta-analyses recently confirmed that metformin therapy reduces the incidence of cancers (including colorectal cancer, hepatocarcinoma, breast cancer) and cancer-related mortality. Metformin may exert its anti-cancer activity by a direct effect (insulin) and an indirect effect (AMPK and mTOR). Considering all promising clinical information in patients with type 2 diabetes, further clinical trials are currently ongoing with the aim of assessing the role of metformin in oncology, especially as adjuvant in breast cancer therapy.