Accumulating epidemiological evidence shows that being either overweight or obese, in other words having excess body weight (EBW), is associated with an increased risk of several, common, adult cancers. The molecular mechanisms that underlie these associations are not understood fully, but insulin resistance is likely to be important. The insulin-cancer hypothesis postulates that chronic hyperinsulinemia is associated with decreased concentrations of insulin-like growth factor binding protein1 (IGFBP-1) and IGFBP-2, leading to increased availability of IGF-I and concomitant changes in the cellular environment that favor tumor formation. However, the situation is likely to be more complex because hyperinsulinemia is also associated with alterations in related molecular systems (e.g. sex steroids and adipocytokines). As the prevalence of EBW increases to epidemic proportions, untangling the links between EBW and the insulin-IGF axis and its wider molecular interactions will become increasingly important in the development of preventive strategies.