Without regular supplies of some hormones our capacity to behave would be seriously impaired; without others we would soon die. Tiny amounts of some hormones can modify our moods and our actions, our inclination to eat or drink, our aggressiveness or submissiveness and our reproduction and parental behavior. And hormones do more than influence adult behavior; early in life they help to determine the development of bodily form and may even determine an individual's behavior capacities. Later in life the changing outputs of some endocrine glands and the body's changing sensitivity to some hormones are essential aspects of the phenomena of aging.

Communication within the body and the consequent integration of behavior were considered the exclusive province of the nervous system up to the beginning of the present century. The emergence of endocrinology as a separate discipline can probably be traced to the experiments of Bayliss and Starling on the hormone secreting. This substance is secreted from cells in the intestinal walls when food enters the stomach: it travels through the bloodstream and stimulates the pancreas to liberate pancreatic juice, which aids in digestion. By showing that special cells secrete chemical agents that are conveyed by the bloodstream and regulate distant target organs or tissues, Bayliss and Starling demonstrated that chemical integration can occur without participation of the nervous system.

The term "hormone" was first used with reference to secreting. Starling derived the term from the Greek “hormon”, meaning "to excite or set in motion." The term "endocrine" was introduced shortly thereafter. "Endocrine" is used to refer to glands that secrete products into the bloodstream. The term "endocrine" contrasts with "exocrine," which is applied to glands that secrete their products through ducts to the site of action. Examples of exocrine glands are the tear glands, the sweat glands, and the pancreas, which secretes pancreatic juice through a duct into the intestine. Exocrine glands are also called duct glands, while endocrine glands are called ductless.