Pain is a complex biopsychosocial phenomenon that most human beings experience at different times throughout their life span. This phenomenon has perplexed man for centuries, and for centuries pain has been poorly managed. Today, health care professionals are placed in a position where patients rely on them to provide pain control, and effective pain management is important in enabling patients to progress in their rehabilitation and to have an improved quality of life. This entry reviews definitions of pain, summarizes the demographics and epidemiology of pain, and describes the physiology of pain and its categorization. It also considers the interventions available, as well as some of the ethical issues that arise with respect to pain treatment. Ancient civilizations, including early Mesopotamia, Egypt, China, Greece, and Rome, used various primitive approaches to treat pain. Via writings, carvings, and other documents, anthropologists have found evidence of pain interventions, including ancient pharmacopoeia such ...