This symposium, the second of an annual series on aging, was held July 5-7, 1955, under the auspices of the Ciba Foundation of London. The aging of transient tissues, that is, those tissues having a shorter normal life than that of the organism as a whole, is considered from a variety of aspects. Although much of the material presented is of theoretical interest, that concerning metabolic changes in the placenta with aging, the effects of growth hormone (somatotropic hormone) in the causation of fetal gigantism, and the changes produced in the offspring of rats whose mothers are treated with cortisone during gestation may have direct clinical implications in pediatrics. On the whole, however, the book is of major interest as it pertains to aging in the placenta, the reproductive organs, and the red blood cell.