THE CHALLENGES OF PAIN ASSESSMENT The ability to recognize and quantify the signs of pain is important to the development of effective pain relieving strategies. The International Association for the Study of Pain has defined pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage1, from which has been extrapolated the following definition for animals: ‘Pain in animals is an aversive sensory and emotional experience (a perception), which elicits protective motor actions, results in learned avoidance, and may modify species-specific traits of behavior, including social behavior’. Pain depends on the activation of a discrete set of receptors and neural pathways and is usually or potentially noxious (that is, harmful, damaging to tissue)2. Pain is a complex phenomenon involving pathophysio - logic and psychological components that are commonly difficult to recognize and interpret in animals. In human patients, pain is what the patient says it is; in animals, pain is what the observer says it is (Fig. 5)!