Pain is a common experience of the ICU patient, with a diverse clinical manifestation. To manage pain we must understand its anatomic pathways and physiology. This article reviews the development of our understanding of the theory of pain from Descartes to the gate theory of Melzack and Wall. We will review the anatomy of the pathways of pain and the interrelationship of "A" and "C" fibers and the unique nature of the opiate receptor.