Many experiences of early life affect the biological characteristics of the adult in a lasting manner. This phenomenon has been illustrated by epidemiological observations in man and by several experimental models in mice. It has been shown, for example, that when newborn animals are nursed by mothers fed diets that are slightly inadequate, their size remains subnormal throughout their life span, even though the young are fed an optimum diet after weaning. A similar depression of growth can be produced by subclinical infections shortly after birth. Decrease in resistance to various forms of stress can be brought about in young animals by various types of nutritional and environmental disturbances so mild that their effects are not recognized when the animals are maintained under usual laboratory conditions. These findings indicate the possibility of devising laboratory models for the analysis of many puzzling sociomedical problems.