Carotid artery stenosis refers to atherosclerotic narrowing of the extracranial carotid arteries. It typically refers to the internal carotid arteries or the common and internal carotid arteries. A "clinically important" degree of stenosis is defined as the percentage of stenosis that corresponds to a substantially increased risk for stroke. However, because stroke risk depends on more than just the degree of stenosis, it is difficult to set a lower limit on the range that defines potential clinical importance. Asymptomatic patients have no significant neurologic symptoms referable to the carotid artery and have not experienced a cerebrovascular event (i.e., a stroke or transient ischemic attack). The main purpose of this report is to systematically evaluate the current evidence on whether screening asymptomatic adults for CAS reduces the risk for fatal or nonfatal ipsilateral stroke and the evidence on harms associated with screening and interventions for CAS. The scope and methods of this report differ from earlier USPSTF reviews on this topic by 1) using systematic methods for all key question, 2) addressing new key questions about the availability of valid, reliable risk stratification tools to distinguish a person's likelihood for asymptomatic CAS and to distinguish risk for ipsilateral stroke caused by CAS or for harms from surgery or intervention in persons with asymptomatic CAS (recommendations of some groups state that screening might be considered for persons with multiple risk factors), 3) adding carotid angioplasty and stenting (CAAS) to the included interventions, 4) adding a question about the incremental benefit of medical therapy for asymptomatic CAS, and 5) conducting quantitative synthesis for many outcomes.