This is a compilation of the various papers presented at the conferences on aging arranged by the Josiah Macy, Jr., Foundation. The participants include persons interested in medical problems related to aging, but many of them are not gerontologists in the strict sense. Such subjects as regeneration of the liver in old age, sexual regeneration in elderly women, problems encountered in an old age counseling center, problems associated with age of retirement and nutritional factors are among those discussed. This book should be useful to those interested in the problem of aging. The discussions on the animal studies are better than those on studies concerned with man.