A normal functioning reproductive endocrine system is a prerequisite for normal male fertility. Any disruption of the delicately coordinated interaction between the components of the hypothalamic-pituitary-testicular axis may lead to hypogonadism and/or infertility. The goal of the clinical evaluation is to determine if the patient has an abnormality of testosterone production or action, the etiology of the abnormality, and if hormone therapy will correct the infertility. Based on a careful history, physical examination, and evaluation of the hormones of the reproductive axis, the physician will ascertain if the patient's hypogonadism is (1) prepubertal or postpubertal in onset; (2) the result of an abnormality in the hypothalamic-pituitary axis, the testes, or the androgen receptor; or (3) associated with another underlying medical condition. This information will place the patient into one of four diagnostic categories: hypogonadotropic hypogonadism, testicular failure, 5alpha-reductase deficiency, or androgen resistance. Within each category are disorders with identifiable pathogenic mechanisms. Recent studies have added to these lists and have provided insights into the molecular basis and inheritance patterns of several of these endocrinopathies.