One of the most frequent, but also most underdiagnosed, endocrinopathies is male hypogonadism (testosterone deficiency). Male hypogonadism presents with a vast clinical picture that is most often associated with typical symptoms, such as disturbances of mood as well as sexual functions. Further on, a decrease in muscle mass and strength, an accumulation of body fat and osteopenia/osteoporosis are frequently observed. Restoring serum testosterone levels by replacement therapy can markedly mitigate, if not totally relieve, the clinical picture of hypogonadism.New treatment modalities have been introduced during the recent years, which include short-acting transdermal modalities as well as the long-acting depots of testosterone.This book gives detailed and up-to-date information regarding the various modern methods of diagnosis of andrological pathologies, initiation and surveillance of testosterone substitution therapy and/or other hormone substitution modalities.The book pays special attention to the ageing patients and their specific needs and we are happy to provide the reader with a vast range of case discussions, to facilitate the transfer of theoretical and recent knowledge into daily clinical practice.