The aging process in men is accompanied by a progressive decline in serum testosterone levels. In addition to chronological aging, the various illnesses occurring in mid- to late-adult life and the medications used to treat them further contribute to lowering circulating testosterone levels. Any acute severe illness produces hypogonadotrophic hypogonadism; but also chronic illnesses, such as diabetes, cardiovascular disease and hypertension, lifestyle habits, such as tobacco and alcohol intake, and nutritional factors, from malnutrition to obesity, account for and accentuate the age-related decline in serum androgen levels. These issues are reviewed in a clinical perspective.