The practice of assessing and adjudicating competence for criminal adjudication in the United States developed largely without assistance from the U.S. Supreme Court or other state and federal courts of appeal for most of the nineteenth and twentieth centuries. However, the need for appellate guidance became evident in the 1980s, especially regarding the significance of mental or emotional conditions that can impair capacity for rational decision-making of persons accused of criminal behavior. During the past thirty years, some governing principles have come into view, but important issues remain unresolved. After a brief review of the historical and conceptual foundations of the competence requirement, this article focuses on two decisions in which the Supreme Court has addressed decisional competence. In Godinez v. Moran (1993), the Court ruled that a pretrial finding that a defendant was competent to stand trial established that he was competent to waive representation by counsel and plead guilty because the test for competence is the same in the two contexts. However, in Indiana v. Edwards (2007), the Court held that a defendant who was found competent to stand trial while being represented by counsel may nonetheless be found to be incompetent to represent himself at trial. Although these decisions are not strictly contradictory, they are in deep tension with one another. This article attempts to set the law on a coherent path by highlighting the significance of doubts about decisional competence in both cases and formulating a coherent approach to guide state and federal courts in the future. In so doing, it builds on the influential empirical contributions of the MacArthur Foundation Research Network on Mental Health and the Law and integrates the author’s writing on this subject that was grounded in the MacArthur Network’s deliberations and has evolved over three decades.