Even though it is a costly, poorly understood problem, very little effort has been dedicated toward research on the mechanisms of acute postoperative pain. Presumably, if we learn more about the etiology of acute incisional pain and the sensory processes that intensify pain after surgery, new treatment methods can be advanced. Since the mid-1990s, our group and others have developed and characterized models for postoperative pain. In one model, a hind paw plantar incision is made. Persistent reduced withdrawal thresholds to mechanical stimulis suggesting hyperalgesia are present. No tonic or spontaneous pain is apparent, but diminished weight bearing is noted on the incised hind paw. Pain-related behaviors remain remarkable for several days and then gradually decrease. The model therefore has similarities to the time course for pain in postoperative patients. Ovariohysterectomy, a clinically relevant procedure, has been used to study postoperative pain. Both rat and dog models have been studied, and a variety of pain-related behaviors including pain at rest and wound sensitivity have been examined. These models will improve our understanding of unique pain mechanisms caused by particular injuries. As we understand postoperative pain and determine the unique mechanisms for acute tissue injury pain, better treatments will evolve and perioperative morbidity will decrease.