Pain is one of the most complex of human experiences. It is the most common reason for which patients seek medical care. More than 80% of all physician visits are due to pain. Pain accounts for over $70 billion annually in health care costs and lost productivity. It is defined by the International Association for the Study of Pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage.” Accordingly, the perception of pain is not only a sensory experience, but also an emotional experience. It is important to distinguish between acute and chronic pain. The distinction between the two is not simply a matter of duration: (1) Acute pain is biologically useful; it serves as the body’s alarm of an underlying medical condition, whereas chronic pain loses this function. (2) The etiology of acute pain is almost always identifiable, whereas the complex interaction of physical and emotional factors in chronic pain make the etiology murky. (3) Cure and relief are almost always attainable in acute pain, but is often not possible with chronic pain; the goal in chronic pain treatment is to improve functionality. (4) Acute pain may lead to anxiety, whereas chronic pain is often associated with depression.