Pain is a complex unpleasant phenomenon composed of sensory experiences that include time, space, intensity, emotion, cognition and motivation. Analgesics are the agents, which selectively relieve pain by acting in the CNS or by peripheral pain mechanisms without significantly altering consciousness. Analgesics may be narcotic or non-narcotic. The study of pain in animals raises ethical, philosophical and technical problems. Philosophically, there is a problem that pain cannot be monitored directly in animals but can only be measured by examining their responses to nociceptive stimuli. The observed reactions are almost always motor responses ranging from spinal reflexes to complex behavior. The animal models employed for screening of analgesic agents, include Pain-state models based on the use of thermal stimuli, mechanical stimuli, electrical stimuli and chemical stimuli. The neuronal basis of most of the above laboratory models is poorly understood, however their application is profitable in predicting analgesic activity of newly discovered substances.