Overall, participation rates in cancer clinical trials are very low, ranging from 3 to 20% of eligible participants. However, participation rates are especially low among the socially disadvantaged and racial/ethnic minority groups that have been historically underrepresented in clinical research. Structural factors such as study duration, treatment or intervention schedule, cost, time, followup visits, and side effects represent more of a barrier to participation among these groups compared with white, non-Hispanics. Attitudes, beliefs, perceptions, and knowledge regarding clinical research, and cultural characteristics of underrepresented minorities pose additional barriers to participation. This article focuses on the structural, cultural, and linguistic factors that affect participation in clinical cancer research for each major U.S. racial/ethnic group. Low socioeconomic status, speaking a primary language other than English, differences in communication styles, mistrust of research and the medical system, fear, embarrassment, and lack of knowledge about the origin of cancer appear to have a negative impact on clinical cancer research participation rates. Much of the information about these factors comes from studies of cancer screening because little data is available on the factors that prevent and facilitate participation of minorities in clinical cancer trials specifically. Such research is needed, and, given the heterogeneity within and between minority populations, should occur in several different geographic settings and with as many different minority subpopulations as possible.