Popular culture often equates testosterone with virility, strength, and the macho male physique. Viewed by some as an antiaging tonic, testosterone’s reputation and increased use by men of all ages in the United States have outpaced the scientific evidence about its potential benefits and risks. In particular there has been growing concern about an increase in the number of middle-aged and older men using testosterone and the lack of scientific data on the effect it may have on aging males. Studies of testosterone replacement therapy in older men have generally been of short duration, involving small numbers of participants and often lacking adequate controls. Testosterone and Aging weighs the options of future research directions, examines the risks and benefits of testosterone replacement therapy, assesses the potential public health impact of such therapy in the United States, and considers ethical issues related to the conduct of clinical trials. Testosterone therapy remains an attractive option to many men even as speculation abounds regarding its potential.