Over the last two decades our insight into testosterone (patho-)physiology has progressed. It is clear that there is an age-related decline of plasma testosterone levels, particularly of non-bound testosterone. In addition, it has become apparent that testosterone (and its metabolic products) have a large number of functions not related to the classical reproductive and sexual actions of testosterone. Hypogonadism can cause osteoporosis, anemia, decrease of lean body mass, increase of body fat content, and a dry skin. There are also a number of psychological complaints such as fatigue, aggressiveness, decrease of cognitive abilities and depression.Professional organizations have formulated guidelines / recommendations for the administration of testosterone to elderly men. This demonstrates that there is a common agreement among experts in regard for the need to define androgen deficiency in various stages of male life.Testosterone exerts its actions via testosterone receptors leading to gene transcription. The higher the number of CAG repeats, the lower the transcriptional activity of the androgen receptor. This mechanism impacts on both effects and side effects of testosterone.Side effects concern mainly the prostate and erythropoeisis, but the currently available literature indicates that there is no increased risk of developing prostate cancer in men receiving testosterone treatment.Following the guidelines as specified by a number of professional organizations, truly testosterone-deficient elderly men can be responsibly treated with testosterone.