Cancer and ageing are both fuelled by the accumulation of cellular damage. Consequently, those mechanisms that protect cells from damage simultaneously provide protection against cancer and ageing. By contrast, cancer and longevity require a durable cell proliferation potential and, therefore, those mechanisms that limit indefinite proliferation provide cancer protection but favour ageing. The overall balance between these convergent and divergent mechanisms guarantees fitness and a cancer-free life until late adulthood for most individuals.