Hormones are chemicals which circulate in the blood stream and spread around the body to carry messages or signals to different parts of the body. The name hormone comes from the Greek word hormao meaning "I excite" and refers to the fact that each hormone excites or stimulates a particular part of the body known as the target gland.

Hormones are made in endocrine glands and passed from the cells of the gland directly into the blood flowing through the gland. Generally, the higher the amount of hormone that is in the blood, the greater the effect its the targets.

The endocrine system consists of glands situated in different areas of the body as shown below. Each gland produces different hormones which regulate the activity of other organs and tissues in the body. These hormones are released directly into the blood flowing through the gland. This is in contrast to exocrine glands, which release hormones down a tube or duct.

The science concerned with the structure and functions of the endocrine glands and the diagnosis and treatment of disorders of the endocrine system is called endocrinology. This term comes from the Greek words endo (within), crine (to secrete) and logos
(study of).