Male hypogonadism is a clinical condition caused by a partial or total breakdown of the cross communication among the hypothalamus, pituitary, and testis. It is generally divided into primary hypogonadism, caused by an abnormality of the testis function, and secondary hypogonadism, due to a pituitary or hypothalamic dysfunction. In primary hypogonadism, the testis is dysfunctional and fails to release sex steroids and sperm, despite strong stimulation by the pituitary (hypergonadotropic hypogonadism), whereas in secondary hypogonadism, the testis is normal but inadequately stimulated by gonadotropins (hypogonadotropic hypogonadism). Despite taxonomy classification, the clinical features of male hypogonadism depend upon the age of onset, the severity of testosterone (T) deficiency, and whether there is an impairment only in terms of sperm production or also in T production. In this chapter, the pathogenesis and clinical characteristics of both primary and secondary hypogonadism, along with their main underlying causes, will be evaluated and discussed in detail.