 Relevance- Failure to prescribe anticoagulant therapy for stroke prevention in AF represents a critical patient safety concern. The investigators successfully applied alert-based CDS to another important unmet patient safety need: prescription of thromboprophylaxis in hospitalized patients at increased risk for VTE. During Year 1 of this study, Brigham and Womens Hospital will design an alert-based CDS program to notify the individual clinician in a private electronic message that his or her hospitalized patient is at risk for stroke due to AF or atrial flutter. The message will specify that there is no order for anticoagulant therapy and will contain the patients predicted risk of developing a stroke in the absence of anticoagulation. In Years 2 and 3, the investigators will conduct a randomized controlled trial to evaluate the impact of an alert-based CDS strategy on prescription of anticoagulant therapy in at-risk patients with nonvalvular AF or atrial flutter. 