Health professionals are routinely exposed to evidence of pain in others. It is important that the processes by which they evaluate pain be understood. The purposes of this article are to review and synthesize recent research on how health professionals judge the pain of others and to present a conceptual model of this process. Methodological and conceptual issues in the conduct of pain judgement studies are addressed. Research in this field over the last 40 years has indicated that, when compared with the pain judgements of patients themselves, health professionals tend to underestimate pain. The authors review the relation of this underestimation bias to such variables as the nature of the patient's pain and the clinical experience of the judge. They also review experiential and cognitive-perceptual variables found to influence the degree of underestimation bias, such as the amount of exposure to evidence of pain and suspicion about the motivations of the patient. A model of the pain decoding process is presented. The issue of whether underestimation has implications for treatment outcome is addressed and priorities for future research are identified.