Recent evidence has shown that fish display aversive behavioral and physiological reactions and a suspension of normal behavior in response to noxious stimuli that cause pain in other animals and humans. In addition to these behavioral responses, scientists have identified a peripheral nociceptive system and recorded specific changes in the brain activity of fish during noxious stimulation. As a result of these observations teleost fish are now considered capable of nociception and, in some opinions, pain perception. From both an experimental and an ethical perspective, it is important that scientists be able to assess possible pain and minimize discomfort that may result from invasive or other noxious procedures. If scientists accept that the definition of pain in animals cannot include direct measurement of subjective experience (the standard for humans), then fish fulfill the criteria for animal pain. In this review, recent evidence for pain is discussed in terms of the physiological properties of nociceptors, central responses to noxious stimulation, and changes in behavior and physiology that are indicative of nociception and are responsive to analgesia. To enable the assessment of potential pain, there are descriptions of newly identified robust indicators and species-specific responses that are easily measurable. The article concludes with a discussion of humane endpoints and of the need for alleviation of pain through the use of analgesia and anesthesia.

• S19809/BB_/Biotechnology and Biological Sciences Research Council/United Kingdom