In order for boys and girls to develop properly into men and women and be able to reproduce, hormones (from the Greek hormaein, to arouse) are needed to stimulate their bodies to change. In both boys and girls, hormones cause the sweat and oil glands to become more active, with body odor and acne as results. Pubic and underarm hair starts to grow and teens go through a growth spurt. When the time comes for changes of puberty to occur, the brain releases gonadotropin-releasing hormone (GnRH), which targets the pituitary gland. The pituitary gland, also known as the Master Gland due to its stimulation of other endocrine glands in the body, releases luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

In boys, LH is known as ICSH (interstitial-cell-stimulating hormone) and it stimulates the interstitial cells to produce hormones in the testicles. Although both males and females have androgen and estrogen circulating in their bodies, with low levels of both until puberty, when puberty begins, males begin to have more androgen, including testosterone, and females have a much higher level of estrogen. The increased release of FSH and LH/ICSH signals the development of the secondary sex characteristics of facial hair growth in males and the beginning of the maturation of eggs in the ovaries, resulting in the menstrual cycle. These hormones also cause the shaping of the body into a taller, more angular shape for males and a softer, rounder shape for females.

If the gonads (testes or ovaries) fail to develop, the secondary sex characteristics don't develop either. Music lovers in 18th-century Europe came up with the idea of castrating young boys who had beautiful, clear voices and training them to sing as adult male sopranos. Indeed, some of these boys did become rich and famous from singing in operas, but they were never able to father children of their own.