Restricting food intake to 50 to 70% of that eaten by ad lib-fed rats and mice markedly increases longevity, retards age-associated physiological deterioration, and delays and, in some cases, prevents age-associated diseases. These actions are due to the reduced intake of calories, and thus the phenomenon has been called the antiaging action of caloric restriction (CR). This article focuses on the possible biological mechanisms underlying the antiaging action. The following three proposed mechanisms are considered in depth: 1) attenuation of oxidative damage; 2) modulation of glycemia and insulinemia; 3) hormesis. The evolution of the antiaging action of CR is also considered. Based on this consideration, a scenario unifying the above mechanisms is presented.