Testosterone therapy is increasingly common in the United States, and many of these prescriptions are written by primary care physicians. There is conflicting evidence on the benefit of male testosterone therapy for age-related declines in testosterone. Physicians should not measure testosterone levels unless a patient has signs and symptoms of hypogonadism, such as loss of body hair, sexual dysfunction, hot flashes, or gynecomastia. Depressed mood, fatigue, decreased strength, and a decreased sense of vitality are less specific to male hypogonadism. Testosterone therapy should be initiated only after two morning total serum testosterone measurements show decreased levels, and all patients should be counseled on the potential risks and benefits before starting therapy. Potential benefits of therapy include increased libido, improved sexual function, improved mood and well-being, and increased muscle mass and bone density; however, there is little or mixed evidence confirming clinically significant benefits. The U.S. Food and Drug Administration warns that testosterone therapy may increase the risk of cardiovascular complications. Other possible risks include rising prostate-specific antigen levels, worsening lower urinary tract symptoms, polycythemia, and increased risk of venous thromboembolism. Patients receiving testosterone therapy should be monitored to ensure testosterone levels rise appropriately, clinical improvement occurs, and no complications develop. Testosterone therapy may also be used to treat hypoactive sexual desire disorder in postmenopausal women and to produce physical male sex characteristics in female-to-male transgender patients.