Though unnecessary for life itself, androgens are essential for the propagation of the species and for establishment and maintenance of the quality of life of males through their support of sexual behavior and function, muscle strength, and sense of well-being. In carrying out its many functions, T acts both as hormone and prohormone. It is an outstanding example of the diverse evolutionary utilization of a primitive informational molecule both among and within species. Not only does T act through the androgen receptor both unchanged and via 5 alpha-reduction, but it acts in tissues with a high aromatase level as an estrogen via the estrogen receptor. Furthermore, DHT, binding to the estrogen receptor, can act as an inhibitor of estrogen action. The products of androgen metabolism may also play active regulatory roles in hematopoiesis and in the regulation of certain hepatic enzymes. Table 3 summarizes the actions of secreted T in males indicating the probable effector hormone. While gross hypogonadism is uncommon, mild androgen insufficiency may be much more frequent, especially in older men, and in those receiving treatment for chronic medical conditions. It is quite possible that such individuals would benefit from appropriate androgen therapy were it available, but the current forms of replacement therapy are not very satisfactory. Better approaches are required. With the exception of a small number of secreted proteins, the products of transcription induced by androgens are not, as yet, known. When the androgen receptor gene is cloned it will be possible to identify androgen-regulated genes and their products. It will then be possible to design agents selectively producing specific desired androgenic effects.