Although the molecular mechanisms of insulin action on many physiological and biochemical processes are far from being completely understood, little doubt exists about the involvement of insulin in tumoural processes. Indeed, increased insulin production (either directly by the tumour in an ectopic fashion, or indirectly by stimulation of the pancreatic secretion) is a common phenomenon during cancer development. Paradoxically, the increased production and circulating levels of the hormone are associated with a decrease in sensitivity which leads to insulin resistance in the non-tumoural tissues, resulting in hyperlipaemia and profound alterations in carbohydrate and lipid metabolism. In addition to these effects on the host, insulin can actually increase the incidence of neoplasias and promote tumour growth. This is probably related to the signaling pathway of the hormone, which shares many elements with that of several growth factors. In addition, insulin signaling directly interacts with that of the ras oncogene. We review the involvement of insulin in cancer from a multidisciplinary point of view, with the aim of encouraging the design of future therapeutic options for cancer treatment.