In contrast to innocuous stimuli, which only have transient effects when applied to the body surface, noxious stimuli generate persistent changes in the nervous system. This nociceptive memory manifests itself most prominently as a post-injury sensitization where, after tissue damage, the avoidance reaction and pain that result from subsequent stimuli are exaggerated and prolonged and can be initiated by low intensity stimuli. Similarities between nociceptive sensitization in mammals (including humans) and the mollusc Aplysia californica suggest that fundamental mechanisms contributing to injury-induced behavioral modifications might be widespread in the animal kingdom.