Despite high prevalence, age-related androgen deficiency has not been fully studied. The relevance of the problem is due to the fact that androgen deficiency is manifested not only by impaired sexual function, but also associated with many age-related diseases (obesity, diabetes, coronary artery disease, osteoporosis and others), worsening their course and prognosis. The book was written by the staff of the Russian Endocrinological Research Center of the RAMS - one of the oldest research institutes in the country, in which the study of the problem of age-related androgen deficiency has been conducted for more than 10 years. In addition to the extensive clinical material accumulated during this period, it provides information on the anatomy and physiology of the reproductive system, as well as the physiological effects of androgens. A separate chapter summarizes modern views on the problem of age-related decrease in testosterone secretion in men. The book discusses in detail the pathogenesis of age-related androgen deficiency, methods of its diagnosis and treatment.