What is a hormone? ‘How hormones work’ defines a hormone as a chemical signal which enables an event in one part of the body to have an effect somewhere else. Hormones make up one of the two great physiological control systems—the other being the nervous system—which keep the functions of the body working together. There are strong, multifunctional connections between the nervous and endocrine systems. As well as only operating inside the body, hormones are affected by internal stimuli and by external events detected by the senses. The endocrine system comprises several interlinked sub-systems, including five main axes centred on the hypothalamic and anterior pituitary gland. The processes of hormone transport and action through these systems are described.