Research on pain in humanshas been animportant clinical topic formany years.Basic science studieswere relatively few in number until experimental work on pain accelerated following detailed descriptions of peripheral nociceptors and central nociceptive neurons that were made in the 1960’s and 70’s, by the discovery of the endogenous opioid compounds and the descending pain control systems in the 1970’s and the application of modern imaging techniques to visualize areas of the brain that are affected by pain in the 1990’s.Accompanying these advances has been the development of a number of animalmodels of human pain states, with the goal of using these to examine pain mechanisms and also to test analgesic drugs or non-pharmacologic interventions that might prove useful for the treatment of pain in humans. Basic research on pain now emphasizes multidisciplinary approaches, including behavioral testing, electrophysiology and the application of many of the techniques of modern cell and molecular biology, including the use of transgenic animals.The “Encyclopedia of Pain” is meant to provide a source of information that spans contemporary basic and clinical research on pain and pain therapy. It should be useful not only to researchers in these fields but also to practicing physicians and other health care professionals and to health caree ducators and administrators.The workis subdivided into 35 Fields, and the Field Editor of each of these describes the areas covered in the Fields in a brief reviewchapter. The topics included in a Field are the subject of a series of short essays, accompanied by key words, definitions