In the human male, testosterone is the major circulating androgen. More than 95% of circulating testosterone is secreted by the testis with a production rate of 6-7 mg/day. The clinical effects of androgens are numerous, and testosterone deficiency is associated with a number of clinical abnormalities. Overt hypogonadism results in reductions in bone mineral density, alterations in body composition and effects on mood, aggressive behaviour, cognitive function, sexual function and several factors important for cardiovascular risk. Androgen replacement in this context is clearly beneficial, and numerous studies have demonstrated improvements in bone and muscle mass, reductions in body fat, and positive effects on quality of life following treatment. The benefits of therapy in men with milder degrees of hypogonadism, and elderly men with "physiological" testosterone deficiency, are less clear-cut, and the appropriate biochemical cut-off below which replacement should be offered has not been clearly defined. Several options are available for androgen replacement in adult men. Oral testosterone, intramuscular injections, subcutaneous implants and transdermal therapy have all been used. Each mode of delivery has advantages and drawbacks and the choice between them will often depend on patient reference. Recent advances include the development of longer-acting intramuscular preparations, which offer more stable androgen levels with fairly infrequent injections, and testosterone gel which appears to provide transdermal replacement without a high incidence of skin reactions. This article will examine the evidence concerning the impact of male hypogonadism and the response to androgen therapy. The question of who to treat will be addressed with particular reference to mild hypogonadism and hypogonadism in the elderly. Finally, an overview of the different modes of replacement therapy will be presented. Copyright 2001 S. Karger AG, Basel