By biological age is understood that state of the organism, with the inclusion of both physical and psychic parameters, which has been reached at a certain point of the chronological age. Experiments are reported which have been conducted in an attempt to measure the biological age of human beings by the aid of batteries of tests. So far, more than 1000 subjects have been examined, and they include both "normal subjects" and groups of patients showing such risk factors as overweight, hypertension, and diabetes mellitus. The motivation for such studies is the desire to satisfy practical needs. To tackle this problem it is necessary that investigators representing the various disciplines of gerontology should work closely together.