PUBERTY IS the gradual period of transition between childhood and adulthood during which a growth spurt occurs, secondary sexual characteristics appear, fertility begins, and profound psychological changes take place. Substantial changes in the endocrine systems occur during puberty, but these begin subtly. Although they are reflected in the attainment of mature physical features of sexual development and in pronounced growth, it may be difficult to ascertain when the first changes occur. The easiest one to determine is the initial growth of coarse, pigmented sexual hair. However, the appearance of pubic hair may not indicate the onset of gonadal activity but instead reflect adrenal androgen secretion. The criteria that more accurately reflect gonadal activity are breast development in girls and genital growth in boys. Both are difficult to ascertain. Physicians do not consistently agree on what constitutes the first evidence of breast development, particularly in girls who have considerable subcutaneous tissue in the chest. Because male genital size varies based on multiple factors, it is not always possible to determine if the size observed is due to pubertal hormone stimulation, normal variation in genital size, or the physiological status of the genitals at the instant of observation. Often, by the time physical characteristics clearly indicate the beginning of this transitional period, puberty as assessed by hormonal measurements of the hypothalamic pituitary gonadal axis is well under way. Physical findings alone, especially one as subjective as Tanner stage 2 genitalia, are a poor index of puberty onset.