Delayed adolescence has several causes. Most frequent is the physiological or constitutional (hereditary) delay of growth and adolescence. This is a normal variation of growth and development, with growth, bone age and puberty retarded in a harmonious way. It carries a good prognosis with late but normal puberty and late catch-up growth leading to normal adult height. It manifests itself long before puberty by short stature and retarded bone age. True endocrine defects with permanent hypogonadism (hypothalamic-pituitary deficiency of the gonadotropinds and primary gonadal failure) are rare. Differential diagnosis before puberty is not always possible on clinical grounds alone. The most useful laboratory test consists of the LH-RH test. The i.v. injection of the recently introduced hypothalamic LH-releasing hormone, LH-RH, is followed by an age-dependent increase of the plasma gonadotropinds LH and FSH. This test allows differentiation, before puberty, between constitutional delay of growth and adolescence with a normal response for bone age, true hypothalamic-pituitary insufficiency with no response, and primary gonadal failure with an increased response. True hypogonadism requires permanent sex hormone replacement therapy. Constitutional delay of growth and adolescence in boys may present a psychosocial indication for temporary hormone therapy with testosterone.