As a matter of practical reality, what role patients will play in decisions about their health care is determined by whether their clinicians judge them to have decision-making capacity. Because so much hinges on assessments of capacity, clinicians who work with patients have an ethical obligation to understand this concept. This article, based on a report prepared by the National Ethics Committee (NEC) of the Veterans Health Administration (VHA), seeks to provide clinicians with practical information about decision-making capacity and how it is assessed. A study of clinicians and ethics committee chairs carried out under the auspices of the NEC identified the following 10 common myths clinicians hold about decision-making capacity: (1) decision-making capacity and competency are the same; (2) lack of decision-making capacity can be presumed when patients go against medical advice; (3) there is no need to assess decision-making capacity unless patients go against medical advice; (4) decision-making capacity is an "all or nothing" phenomenon; (5) cognitive impairment equals lack of decision-making capacity; (6) lack of decision-making capacity is a permanent condition; (7) patients who have not been given relevant and consistent information about their treatment lack decision-making capacity; (8) all patients with certain psychiatric disorders lack decision-making capacity; (9) patients who are involuntarily committed lack decision-making capacity; and (10) only mental health experts can assess decision-making capacity. By describing and debunking these common misconceptions, this article attempts to prevent potential errors in the clinical assessment of decision-making capacity, thereby supporting patients' right to make choices about their own health care.