SIGNIFICANT QUALITY-OF-CARE GAPS ARE WELL DOCUmented in the United States. These reports have focused mostly on underuse of performance measures of important processes of care, and some outcomes of care. Others have argued that the cause of underuse of these evidence-based processes of care is usually not deficient physician knowledge about whether to perform the examination or order the test, but rather poorly designed, dysfunctional microsystems of care unable to deliver effective, efficient, and reliable care. Consequently, much of the recent work in quality improvement has focused on changing microsystems of care “to deliver the right care for the right patient at the right time, all the time.” What is often overlooked in quality improvement, but equally important, is that effective microsystems must have highly competent clinicians, who possess sufficient knowledge and clinical skills to make and execute evidence-based decisions, exercise informed clinical judgment, and deal effectively with uncertainty. Clinical judgment and the ability to deal with uncertainty are especially critical with respect to misuse and overuse of processes of care. Misuse and overuse of processes of care (eg, overprescribing antibiotics and unnecessary imaging and procedures) put patients at greater risk for unnecessary complications. Physician knowledge and clinical judgment also are central to making correct diagnoses. The majority of current performance measures assume a correct diagnosis, but more than that current measures cover only a fraction of the myriad health problems seen by physicians on a daily basis and likely will never address unusual or less common but no less important or serious conditions. Furthermore, many symptoms and signs that prompt patients to see physicians are often not well-defined and a diagnosis often remains uncertain after the initial visit. Clinical judgment is crucial in determining when further intervention is necessary or when watchful waiting may be the best approach. Even when an accurate diagnosis is made, prudent clinical judgment is necessary to determine appropriate care, including the correct diagnostic tests, critical to the efficiency and effectiveness aspects of quality. Our objectives in this Commentary are to discuss the relationship between medical knowledge and quality and how the secure examination component of specialty board certification—with its primary focus on assessing physician knowledge, diagnostic acumen, and clinical judgment—is an important complement to current performance measures. Recognizing this importance, in 2006 the American Board of Internal Medicine instituted a new requirement for all physicians with time-limited certificates to evaluate their performance in practice to address physician competence in practice-based learning and improvement and systemsbased practice. We hope this discussion will stimulate dialogue about the need for more comprehensive physician performance measurement in the era of public reporting.