Puberty is the period during which growing boys or girls undergo the process of sexual maturation. Puberty involves a series of physical stages or steps that lead to the achievement of fertility and the development of the so-called secondary sex characteristics, the physical features associated with adult males and females (such as the growth of pubic hair). While puberty involves a series of biological or physical transformations, the process can also have an effect on the psychosocial and emotional development of the adolescent.

Doctors do not completely understand the timing of the onset of puberty; a number of factors likely determine its onset. One theory proposes that reaching a critical weight or body composition may play a role in the onset of puberty. The increase in childhood obesity may be related to the overall earlier onset of puberty in the general population in recent years.Leptin, a hormone produced by fat cells (adipocytes) in the body, has been suggested as one possible mediator of the timing of puberty. In research studies, animals deficient in leptin did not undergo puberty, but puberty began when leptin was administered to the animals. Further, girls with higher concentrations of the hormone leptin are known to have an increased percentage of body fat and an earlier onset of puberty than girls with lower levels of leptin. The concentration of leptin in the blood is known to increase just before puberty in both boys and girls.

Leptin, however, is likely only one of multiple influences on the hypothalamus, an area of the brain that releases a hormone known as gonadotropin-releasing hormone (GnRH), which in turn signals the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH secretion by the pituitary is responsible for sexual development through regulation of the production of estrogen and testosterone. Genetic factors are likely involved in the timing of puberty, and the timing of puberty tends to "run in families." Additionally, a gene has been identified that appears to be critical for the normal development of puberty. The gene, known as GPR54, encodes a protein that appears to have an effect on the secretion of GnRH by the hypothalamus. Individuals who do not have a functioning copy of this gene are not able to enter puberty normally.

The onset of puberty varies among individuals. Puberty usually occurs in girls between the ages of 10 and 14, while in boys it generally occurs later, between the ages of 12 and 16. In some African-American girls, puberty begins earlier, at about age 9, meaning that puberty occurs from ages 9