Our life, bounded by birth and death, has five chief stages, each of which, while it may be divided into substages, also passes into the next so gradually that we cannot date, save roughly and approximately, the transition from one period to that which succeeds it. These more marked nodes in the unity of man’s individual existence are: (1) childhood, (2) adolescence from puberty to full nubility, (3) middle life or the prime, when we are at the apex of our aggregate of powers, ranging from twenty-five or thirty to forty or forty-five and comprising thus the fifteen or twenty years now commonly called our best, (4) senescence, which begins in the early forties, or before in woman, and (5) senectitude, the post-climacteric or old age proper. My own life work, such as it is, as a genetic psychologist was devoted for years to the study of infancy and childhood, then to the phenomena of youth, later to adulthood and the stage of sex maturity. To complete a long-cherished program I have now finally tried, aided by the first-hand knowledge that advancing years have brought, to understand better the two last and closing stages of human life. In fact ever since I published my _Adolescence_ in 1904 I have hoped to live to complement it by a study of senescence. The former could not have been written in the midst of the seething phenomena it describes, as this must be. We cannot outgrow and look back upon old age, for the course of time cannot be reversed, as Plato fancied life beginning in senility and ending in the mother’s womb. The literature on this theme is limited and there are few specialists in gerontology even among physicians. Its physiological and pathological aspects have been treated not only for plants and animals but for man, and this has been done best by men in their prime. For its more subjective and psychological aspects, however, we shall always be dependent chiefly upon those who are undergoing its manifold metamorphoses and therefore lack the detachment that alone can give us a true and broad perspective.