Researchers, practitioners, and participants in cancer clinical trials must have a clear understanding of clinical trials if participation in them is to be solicited ethically and effectively. A valid and reliable measure of cancer clinical trial understanding did not exist prior to a 2005 study conducted for the Coalition of Cooperative Cancer Groups. This report outlines a measure derived from that study, discusses the rationale for its component items, examines its psychometric properties, and demonstrates the relationship of this measure to the enrollment decision. Data from national samples of cancer survivors and the general public demonstrate the measure's validity and reliability. Results are discussed as they relate to patient understanding of clinical trials, informed decision making, and health communication processes.