1. By means of the twin family method and up to the end of 1947, a representative sample of 1,602 twin index cases over 6o years of age was collected from both the general and institutionalized populations of the state of New York, in order to study the biological, social, and psychiatric problems of senescence under experiment-like, but mankind-specific conditions. The study was organized in such a way that complete life histories can be obtained for the purpose of analyzing the differences observed between one-egg and two-egg twin groups with regard to length of life, senile manifestations, and causes of death.2. The random sampling of the series of aged twins included in this preliminary analysis is indicated not only by the sex distribution of the index cases (697 males to 905 females), but also by the obtained ratio of monozygotic to dizygotic pairs (237 to 548).3. Many basic elements in the physical and psychological similarities of one-egg twin partners have a tendency to persist throughout life and definitely exceed those found in two-egg pairs, frequently withstanding advancing age as well as significant environmental diversity. Intellectual abilities have also been shown psychometrically to depend on genetically produced potentialities, which in their measurable effects remain variable from man to man until senescence.4. Married one-egg twin partners of either sex differ less in their rates of reproductivity than do same-sexed dizygotic twin pairs. This finding disagrees with the popular notion that the female partner is the chief determining factor in the regulation of family size.5. In comparing the life spans of same-sexed twins, the average intra-pair difference of monozygotic pairs has been found to be only one-half that of dizygotic pairs, indicating the significance of genetic factors in longevity. The total mean difference is 36.9 months in the monozygotic group, and 78.3 months in the same-sexed dizygotic group.6. The biology of senescence cannot be clearly understood without reference to life itself in terms of the variable capacity for survival.