 The purpose of this study is to develop and test an electronic clinical decision support system, CDSS, to standardize the management of adults with atrial fibrillation, AF, or atrial flutter in the emergency department, ED. The CDSS will provide evidence-based recommendations to emergency physicians at the point of care, aiming to reduce variation in management, preventable hospitalizations, and stroke risk. This study will use a step-twedge cluster randomized pragmatic clinical trial design to evaluate the impact of the CDSS on hospitalization of adults with AF or atrial flutter presenting to the ED and on anticoagulation prescription ineligible patients at the time of ED discharge and within 30 days. This article was offered by David R. Vincent, Adina S. Rauchwerger, John Dewey Karadi, and others.