In Dusky v. United States, the United States Supreme Court established "rational understanding" as a necessary component of a defendant’s competency to stand trial. Yet, rational understanding has engendered misunderstanding, stemming from inconsistent court rulings and lack of systematic attention given to definitions of rationality. The purpose of this article is to assist with the conceptualization of rational understanding as it relates to competency to proceed to trial. This will be accomplished through a review of legal decisions and scholarly papers that provide various definitions of rationality. We discuss the suitability of standardized instruments of competency and how they may assist in providing a valid metric for evaluating rational abilities. We also provide discussion of how case law, in conjunction with psycholegal research, can be used to gain nuanced insight into operationalizations of rational understanding. By gaining a thorough understanding of rationality in competency to proceed to trial evaluations, clinicians may improve on the quality and foundation of their evaluations.