The physical and hormonal changes of puberty are presented and the wide range of ages at which the pubertal process may begin is emphasized. The great variability in the timing of onset of adolescence, its rate of progression, and the age of completion are detailed. The causes of delayed adolescence in males and females are considered. The most common form of delayed adolescent development is termed constitutional delay in growth and development, which may occur sporadically, or may be the familial pattern of growth and development or may reflect a suboptimal nutritional environment. The evaluation of such children, including appropriate historical review, physical examination, and laboratory assessment, is outlined. In most patients with constitutional delay in growth and development, strong reassurance is sufficient therapy. In other subjects, treatment with androgens (boys) or estrogens (girls) may be indicated. In patients with primary systemic diseases accociated with delayed maturation, specific treatment which eradicates the illness will often be followed by resumption of growth and development. In subjects with primary disorders of the hypothalamus, pituitary, or gonads, replacement therapy with androgens or estrogens is indicated. If gonadal function is intact, these patients may eventually become fertile with appropriate use of hypothalamic and/or pituitary hormones.