Word "pain" derives its origin from the Indo-European root aleg meaning to suffer. Word "pain" is later and comes from the Latin word "poena" meaning punishment. Since ancient times there has been disagreement regarding the perception of pain and its evaluation. Unlike sight, hearing and smell, pain does not seem to be a primary sensation, but rather an emotional experience. Most pain researchers view pain as a complex perception, induced by noxious stimuli. Although pain is the most frequent symptom in medicine and despite the enormous advances that have been made in the field of analgesia and anesthesia, the pathophysiological mechanisms involved in its generation and maintenance are not fully understood. Definition of pain was given in 1979 by the classification committee of the international association for the study of pain (IASP) "as an unpleasant aesthetic and emotional experience, associated with actual or potential tissue damage or described in terms of such damage". In other words, although physiology and anatomy determine a precise point of reference for the detection and transmission of messages interpreted as painful, what differentiates the experience of pain is the fact that there is always an emotional gradient to the experience of pain. The purpose of the review is to investigate the analgesic system. Pain signals can be blocked at their initial point of entry into the spinal cord. Analgesia system may also inhibit pain transmission elsewhere in the nociceptive pathway. Because most drugs that alter neuronal excitability act on synaptic receptors, it has been suggested that the "morphine receptors" of the analgesia system must actually be receptors for some morphine-like neurotransmitter that is secreted normally from the brain.