Measuring testosterone levels became easier in the 1970s, and it wasn't long before levels were being checked in men across all age groups. At that time, several authors reported an age-associated decline of serum testosterone levels beginning in the fourth or fifth decades of life. Other studies found that the decline in testosterone with age might be more related to comorbidities that develop in many aging men. Aggressive marketing campaigns by pharmaceutical companies have led to increased awareness of this topic, and primary care physicians are seeing more patients who are concerned about "low T." Unfortunately, testosterone replacement therapy has not been straightforward. Many men with low testosterone levels have no symptoms, and many men with symptoms who receive treatment and reach goal testosterone levels have no improvement in their symptoms. The actual prevalence of hypogonadism has been estimated to be 39% in men aged 45 years or older presenting to primary care offices in the United States. As the US population ages, this number is likely to increase. This article, targeted to primary care physicians, reviews the concept of late-onset hypogonadism, describes how to determine the patients who might benefit from therapy, and offers recommendations regarding the workup and initiation of treatment.