Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in adults, with lifetime rates of 21% to 33%. There are numerous risk factors, including older age, hypertension, coronary disease, obstructive sleep apnea, diabetes, and others. Patients engaging in lifelong high-endurance exercise also have increased risk. Some organizations recommend screening; others do not. However, many patients identify AF themselves using mobile cardiac monitoring devices, some of which accurately detect the arrhythmia. Patients with AF with hemodynamic instability are treated with immediate synchronized cardioversion. Treatment options for stable patients include scheduled cardioversion, rhythm control with pharmacotherapy, catheter ablation, and rate control with pharmacotherapy. Catheter ablation is increasingly used as first-line therapy, with up to 80% of patients remaining AF-free after one or two ablation treatments, an outcome superior to that with pharmacotherapy. Patients with AF should receive anticoagulation based on the CHA₂DS₂-VASc (Congestive heart failure, Hypertension, Age 75 years or older [doubled], Diabetes, prior Stroke or transient ischemic attack or thromboembolism [doubled], Vascular disease, Age 65 to 74 years, Sex category) score, and also before and immediately after ablation or cardioversion. It is uncertain whether long-term anticoagulation is needed after successful ablation. Atrial flutter (AFL) is the second most common sustained supraventricular arrhythmia. Patients with AFL are at risk of developing AF, and many recommendations for managing AFL are similar to those for AF. The preferred management for AFL is catheter ablation, with success rates exceeding 90%.