This lucid interpretation of the latest scientific information on male reproductive endocrinology provides a thorough understanding of the pathophysiology, clinical characteristics, and treatments for disorders that adversely affect testicular function. The authors focus on the specific causes of gonadotropin deficiency and testicular failure arising from disorders of the brain and the testis and from gene defects and chronic illness. Additional chapters describe the endocrine mechanism for the decline of testicular function with aging, summarize the data supporting the negative impact of environmental toxins on testicular function, and discuss the testicular consequences of exercise and obesity.