Adjudicative competence, more commonly referred to as competence to stand trial, is a highly under-theorized area of law. Though it is well established that, to be competent, a criminal defendant must have a "rational" as well as 'factual" understanding of her situation, the meaning of such "rational understanding" has gone largely undefined. Given the large number of criminal prosecutions in which competence is at issue, the doctrine's instability stands in stark contrast to its importance. This Article argues that adjudicative competence, properly understood, asks whether a criminal defendant has capacity to participate meaningfully in the host of decisions potentially required of her Further, sound assessment of such capacity requires attention to both the cognitive and emotional influences on rational decision-making in situations of personal relevance and risk. The role of emotion has been neglected, both in traditional accounts of decision-making and in assessments of adjudicative competence, and merits particular attention. This Article explores two examples of potentially competence-threatening emotional dysfunction-severe psychiatric mood disorder and organic brain damage-either of which may interfere unreasonably with decision-relevant emotional perception, processing, and expression. Existing legal theory and forensic testing methods, which reflect a predominantly cognitive approach, do not account adequately for such dysfunction. Shifting the adjudicative competence inquiry away from a general search for "rationality" and toward a more finely grained examination of the cognitive and emotional influences on rational decision-making processes offers our best hope for giving meaning to "rational understanding."