How to Pool Risks across Generations makes the case for the collective provision of pensions, on fair terms of social cooperation. Through the insurance of a mutual association which extends across society and over multiple generations, we share one another's fates by pooling risks across both space and time. Resources are transferred, not simply between different people, but also within the possible future lives of each person: from one's more fortunate to one's less fortunate future selves. The book opens with an investigation of the longevity and investment risk that even a single individual on a desert island would face in providing for her old age. From this atomistic starting point, it builds up, within and across the chapters, to increasingly collective forms of pension provision. By joining together, it is possible to tame the risks we would face as individuals each with our own private pension pot. A collective pension can be justified as a 'social union of social unions': an enduring corporate body, which is formed by agreements to pool risks, in a manner that involves reciprocity between the various individuals that constitute the collective. Even though all individuals age and die, a collective pension scheme remains evergreen, as the average age of members remains relatively unchanged, through the influx of new members to replace those who retire. It is therefore possible to smooth risks indefinitely across as well as within generations, to the mutual advantage of each.