As time passes during the first two decades of life, the human body changes continuously and dramatically, until it reaches the mature adult state. Numerous biological and environmental factors regulate and control the course of human growth. Compared to other animals, primates (including humans) experience a prolonged period of physical growth.

Physical development during infancy and childhood follows the cephalocaudal and proximodistal trends. During puberty, growth proceeds in the reverse direction, and sex-related differences in body proportions appear. The best way of estimating a child's physical maturity is to use skeletal age, a measure of development of the bones of the body. Body growth is controlled by a complex set of hormonal secretions released by the pituitary gland and regulated by the hypothalamus. Individual and cultural differences in body size and rate of maturation are influenced by both heredity and environment. Physical growth is an asynchronous process because different body systems have their own unique, carefully timed patterns of maturation.

The human brain achieves its adult size earlier than other organs. Both heredity and early experience contribute to brain organization, and there may be sensitive periods in which appropriate stimulation is necessary. Studies indicate that brain growth spurts coincide with peaks in children's intelligence test performance and major cognitive changes.

Both heredity and nutrition are factors affecting physical growth. Breast-feeding is especially suited to infants' growth needs, and the importance of nutrition is evident in the dietary diseases of marasmus and kwashiorkor. In industrialized countries, obesity is a nutritional problem with health and psychological consequences, and the most effective treatment is family based and focuses on changing behaviors. Infectious disease can combine with poor nutrition to undermine healthy physical development. Nonorganic failure to thrive and deprivation dwarfism illustrate the role of love and stimulation in children's healthy physical growth.

Puberty is a time of dramatic physical change leading to an adult-sized body and sexual maturity. The psychological impact of pubertal events is a product of both biological and social forces. Adolescents' physical changes and their new powers of reasoning may lead to a rise in family tensions, but the conflict that takes place is generally mild. The timing of puberty has a major impact on psychological adjustment. For some adolescent girls, cultural ideals combine with family and individual psychological problems to produce the serious eating disturbances of anorexia nervosa and bulimia. American adolescents receive mixed messages from adults and the larger culture about sexual activity, which contributes to high rates of sexually transmitted disease, and teenage pregnancy, abortion, and parenthood.