Research on testosterone is increasing in many senses; however there is still much controversy regarding its physiology and clinical use. This book addresses these topics, providing a broad over view about testosterone, from its basic fea tures to the most recent evidence of clin ical applicability. Also, 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. Although not essential for survival, testosterone represents the essence of male biological function, being the important testicular androgen in men. Low serum testosterone levels are asso ciated with cardiovascular morbidity, meta bolic syndrome, type 2 diabetes mellitus, atherosclerosis, osteoporosis, sarcopenia, and mortality. Conversely, increased serum levels of testosterone may lead to deleteri ous events.