To be most useful, clinical trials of cancer pain treatments should use pain measures that are both reliable and valid. A great variety of measures are now available that may be used to assess cancer pain. However, there are not yet any clear guidelines for selecting one or more measures over the others. The purpose of this article is to summarize the evidence concerning the validity and reliability of cancer pain measures. One hundred sixty-four articles were identified that provided psychometric data of pain measures among patients with cancer. The results indicate that commonly used single-item ratings of pain intensity are all valid and adequately reliable as measures of pain intensity, although some scales appear to be easier for patients with cancer to understand and to use than others. Multiple-item measures of pain intensity are reliable, but evidence concerning their validity is lacking. There is a paucity of research examining the psychometric properties of measures of cancer pain interference, pain relief, pain site, the temporal aspects of pain, and pain quality. This lack of evidence limits the conclusions that may be drawn concerning the reliability and validity of these other pain measures. Composite measures that combine ratings of pain intensity and pain interference into a single score appear to be both valid and reliable for describing patient populations, although their usefulness in clinical trials may be limited because they can obscure the contributions of intensity and interference to the total score. Proxy measures of cancer pain (pain ratings made by someone other than the patient) may be useful when patients are not able to provide pain ratings, but they should not be used as replacements for patient ratings when patient self-report measures are available. The discussion includes specific recommendations for selecting from among the available pain measures, as well as recommendations for future research into the assessment of cancer pain.