Age is a major risk factor for many cancers. Although this is usually viewed in the context of the cell biology, we argue here that age-associated changes to immunity may also contribute to the age-associated increasing incidence of most cancers. This is because cancers are immunogenic (at least initially), and the immune system can and does protect against tumourigenesis. However, immune competence tends to decrease with age, a phenomenon loosely termed "immunosenescence", implying that decreased immunosurveillance against cancer could also contribute to increased disease in the elderly. This review weighs some of the evidence for and against this possibility.