Male hypogonadism is one of the most common endocrinologic syndromes. The diagnosis is based on clinical signs and symptoms plus laboratory confirmation via the measurement of low morning testosterone levels on two different occasions. Serum luteinizing hormone and follicle-stimulating hormone levels distinguish between primary (hypergonadotropic) and secondary (hypogonadotropic) hypogonadism. Hypogonadism associated with aging (andropause) may present a mixed picture, with low testosterone levels and low to low-normal gonadotropin levels. Androgen replacement therapy in hypogonadal men has many potential benefits: improved sexual function, an enhanced sense of well-being, increased lean body mass, decreased body fat, and increased bone density. However, it also carries potential risks, including the possibility of stimulating the growth of an occult prostate cancer. The benefits of androgen therapy outweigh the risks in men with classic hypogonadism. However, for men with mild hypogonadism or andropause, the balance between benefits and risks is not always clear. Unfortunately, studies to date have included too small a number of patients and have been too short in duration to provide meaningful data on the long-term risks versus the benefits of androgen replacement therapy in these populations. Several products are currently marketed for the treatment of male hypogonadism. Weekly-to-biweekly injections of testosterone cypionate (cipionate) or testosterone enanthate (enantate) are widely used, as they are economical and generally well tolerated. However, once-daily transdermal therapies have become increasingly popular and now include both patch and gel systems. Intramuscular injection of testosterone undecanoate is an attractive new therapy that can be administered quarterly. To confirm an adequate replacement dosage, assessment of clinical responses and measurement of serum testosterone levels generally suffice. For selected men, serial measurement of bone mineral density during androgen therapy might be helpful to confirm end-organ effects. For men aged >50 years, we advocate measurement of hematocrit for detection of polycythemia and a digital rectal examination with a serum prostate-specific antigen level measurement for prostate cancer screening during the first few months of androgen therapy. Subsequently, a hematocrit should be obtained yearly or after changes in therapy, and annual prostate cancer screening can be offered to the patient after a discussion of its risks and benefits.