Adolescence is the transitional stage of life between childhood and adulthood, roughly encompassing the ages 12 to 22. G. Stanley Hall, the father of adolescent psychology, conceptualized it as a period of sturm und drang (storm and stress), but later social scientists (such as Margaret Mead) showed that less than half of adolescents experience this stage as stressful.

The major biological transition that takes place in adolescence is puberty. There are five chief manifestations of puberty that occur during adolescence. The first manifestation is a rapid acceleration of growth. The adolescent growth spurt is the most dramatic growth since infancy, and lasts about 4 1/2 years; girls peak in growth spurt around age 11 years, 8 months; boys peak around age 14. The development of primary sex characteristics (related to reproduction-mature sperm and ova) and secondary sex characteristics (ancillary changes- facial and pubic hair, deepening of voice in boys, breast formation and rounding of hips in girls) are the second and third manifestations. Changes in body composition also occur with differences between the sexes. Boys put on more muscle, and girls begin to see an increase in body fat; which would be necessary for menstruation and reproduction. Finally, adolescents experience changes in their circulatory and respiratory systems as part of puberty; the size and capacity of the heart and lungs increases making adolescents more capable of sustained exercise. Around age 8 or 9, the hypothalamus stimulates the anterior pituitary gland, which in turn stimulates the thyroid, adrenals, ovaries, and testes; these changes are manifested by the time the child is 10 or 11. The system works on a negative feedback loop, when the hormone levels rise too high, the system reacts by slowing down production; when they fall too low, production is sped up. Menarche refers to a girl's first menstruation. The average age of menarche has been decreasing historically, with factors such as nutrition, illness, stress, and genetics affecting the timing.

There is considerable variation in the age at which an adolescent hits puberty. Hitting puberty significantly earlier or later than your peers can have effects on both boys and girls. Early maturing boys tend to have a more positive self-image, are more popular, and more muscular, but are also more likely to be involved in alcohol/drug use and delinquency. Early maturing girls tend to be less popular, have more emotional problems (including low self-esteem), have body-image problems, and engage in delinquency and early sex. Late maturing boys tend to feel excluded, but also have higher intellectual curiosity. Late maturing girls tend to also suffer lowered self-esteem, but also are more likely to participate in higher education.

Why do teenagers seem so much more impulsive, so much less self-aware than grown-ups? Cognitive neuroscientist Sarah-Jayne Blakemore compares the prefrontal cortex in adolescents to that of adults, to show us how typically "teenage" behavior is caused by the growing and developing brain.