Recent advances in cellular and molecular biology have markedly increased our understanding of normal and abnormal hypothalamic–pituitary–testicular function. Like other volumes in the Contemporary Endocrinology series, the goal of Male Hy- gonadism: Basic, Clinical, and Therapeutic Principles is to link current knowledge of basic biology to the practice of medicine. The development of new methods for testost- one replacement has substantially increased the number of men who are seeking to determine whether they are hypogonadal, and who are using testosterone replacement therapy, thus mandating a broader understanding of testosterone deficiency. The chapters of this book were contributed by authors from around the world, and from various scientific and clinical disciplines, who have devoted their careers to the study of the physiology and pathophysiology of the male. Thus, this comprehensive and focused volume is intended for a wide audience encompassing both basic scientists and practicing clinicians. Its scope will provide a wealth of information for students and fellows as well.