Hypogonadism is a condition in which the endogenous secretion of testosterone is either insufficient or inadequate to maintain serum testosterone levels within normal range, and may manifest as a variety of signs and symptoms. Age-related hypogonadism is due to a combination of primary hypogonadism (testicular failure) and secondary hypogonadism (hypothalamic-pituitary axis failure). This review provides insight into the mechanisms resulting in the multifactorial nature of acquired androgen-deficiency, and outlines the current controversy regarding testosterone-replacement therapy in aging males.