Fewer than one in 20 adult patients with cancer enroll in cancer clinical trials. Although barriers to trial participation have been the subject of frequent study, the rate of trial participation has not changed substantially over time. Barriers to trial participation are structural, clinical, and attitudinal, and they differ according to demographic and socioeconomic factors. In this article, we characterize the nature of cancer clinical trial barriers, and we consider global and local strategies for reducing barriers. We also consider the specific case of adolescents with cancer and show that the low rate of trial enrollment in this age group strongly correlates with limited improvements in cancer population outcomes compared with other age groups. Our analysis suggests that a clinical trial system that enrolls patients at a higher rate produces treatment advances at a faster rate and corresponding improvements in cancer population outcomes. Viewed in this light, the issue of clinical trial enrollment is foundational, lying at the heart of the cancer clinical trial endeavor. Fewer barriers to trial participation would enable trials to be completed more quickly and would improve the generalizability of trial results. Moreover, increased accrual to trials is important for patients, because trials provide patients the opportunity to receive the newest treatments. In an era of increasing emphasis on a treatment decision-making process that incorporates the patient perspective, the opportunity for patients to choose trial participation for their care is vital.