Normal pubertal development is characterized by major physical alterations: sexual maturation, changes in body composition, and rapid skeletal growth. Breast development is the first manifestation of puberty in approximately 85% of girls; the normal age for initial breast development is 8 to 13 years. Menarche generally occurs within 2 years of the onset of breast development, with a mean age in American girls of 12.8 years. In boys, the first manifestation of puberty is testicular enlargement; the normal age for initial signs of puberty is 9 to 14 years in males. Pubic hair in boys generally appears 18 to 24 months after the onset of testicular growth and is often conceived as the initial marker of sexual maturation by male adolescents. Skeletal growth is one of the most striking characteristics of puberty. Linear-growth velocity begins to increase in males at genital stage III and pubic-hair stage II, but peak height velocity is not attained until age 14 years in boys and 12 years in girls. Lean body mass, which primarily reflects muscle mass, begins to increase during early puberty in both boys and girls. Fat mass increases during the late stages of puberty in girls. Sex differences in the adolescent growth spurt produce the characteristics sexual dimorphism in shape and proportions seen in young adults.