This article, the 12th in a series on the biological basis of child health, focuses on the endocrine system. This system works alongside the nervous system to regulate the functioning of the human body using chemical mediators called hormones. It is composed of several glands secreting a wide range of hormones that act on target cells in organs and tissues. Various functions of the human body are controlled by the endocrine system, including growth, puberty, metabolism and bone health. This article explores the anatomy and pathophysiology of the endocrine system, the effects of hormonal excesses or deficiencies on the body, and the presentation and management of endocrinopathies commonly seen in children.