The interaction of the endocrine, immune, and nervous systems in modulating each other's activity opens the track for a new way of looking to the effects of aging on each of the three systems. This new approach finds it bases in old times, since evolution shows how we missed for many years a relation between the three systems that was clearly manifested in lower species. In this chapter, the effects of aging on the immune, endocrine and nervous systems are proposed as the consequence of the influences of each system on the others.