Even though research on testosterone is increasing, there is still much controversy regarding its physiology and clinical use. This book provides a broad overview on testosterone, from its basic features to the most recent evidence of clinical applicability. In addition, specific conditions in which testosterone play a pivotal role are discussed in detail, such as hypogonadism, misuse and abuse, puberty, cardiovascular effects and testosterone therapy.  The testes are vital organs for reproduction of the human species, besides being the main source of testosterone production in men. Although not essential for survival, these singular structures represent the essence of male biological function.  Testosterone is the most important testicular androgen in men. Low serum testosterone levels are associated with cardiovascular morbidity, metabolic syndrome, type 2 diabetes mellitus, atherosclerosis, osteoporosis, sarcopenia, and mortality.