Hormones are the chemical messengers of the endocrine system and serve as a communication pathway between sensory and motor organs. Hormones thus mediate changes in physiologi cal and behavioral states in response to environ mental and internal fluctuations. Hormones are synthesized and released by ductless glands, are transported through the circulatory system, and have potent and long-lasting effects on distant target tissues. In this essay, an overview of the endocrine system is provided and the biochem istry, mode of action, and regulation of hor mones in general are discussed. In addition, the role of hormones in the regulation of reproduc tive behavior and the stress response is de scribed.