Race and ethnicity are increasing used in cancer research to assess differences in cancer incidence and response to therapy. In this chapter, we discuss the measurement and methodologic issues that should be addressed to minimize bias and derive valid estimates when performing such assessments. These issues include 1) lack of national standards for race and ethnicity categories; 2) difficulty in comparing race and ethnic categories in longitudinal assessments; 3) broad categorization of race and ethnicity groups that do not provide adequate details for meaningful assessments; 4) inaccuracies in race and ethnicity data collection, and 5) confounding by socioeconomics and other factors. Recommendations for improving race and ethnicity data collection also are discussed.