Hypogonadism in males is a clinical syndrome complex which comprises symptoms with or without signs as well as biochemical evidence of testosterone deficiency. The diagnosis of hypogonadism thus includes both clinical history and examination as well as biochemical assessment of serum testosterone levels. Hypogonadal symptoms depend on the age at onset of hypogonadism, severity of the deficiency, its duration and sensitivity to androgen action. Prepubertal onset results in lack of virilization and pubertal development and produces features such as eunuchoid body proportions and undeveloped secondary sex characteristics. Development of hypogonadism in adult life is characterized by a loss of androgen-dependent functions such as reduction in muscle mass, a shift in body composition towards more adipose tissue, decreased sexual function with diminished libido, depressed mood, loss of psychological energy osteoporosis and several other possible symptoms. The majority of men who suffer from hypogonadism do not have classical endocrine disorders. These men present with concomitant disease such as metabolic syndrome or type 2 diabetes, chronic infections, inflammatory disease, COPD, or cardiovascular disease. All these conditions are associated with a high prevalence of hypogonadism. Pharmacological therapy with opiates and corticosteroids are also known to cause hypogonadism. Hypogonadal symptoms are precipitated at different testosterone levels. Total testosterone levels of less than 8 nmol/l highly support a diagnosis of hypogonadism whereas levels greater than 12 nmol/l are likely to be normal. The grey zone between 8 and 12 nmol/l requires further evaluation and assessment of free or non-sex hormone-binding globulin-bound (bioavailable) testosterone. A trial period of testosterone treatment may be required.