Atrial fibrillation (AF) is the most common form of arrhythmia in the intensive care unit (ICU) and is associated with increased mortality. A total of five types of AF can be distinguished: initially diagnosed, paroxysmal, persistent, long-standing persistent and permanent AF. In addition to the initial treatment, antithrombotic therapy, rate and rhythm management can be used. The treatment of comorbidities is part of the patient management and for patients with increased risk of thromboembolic events anticoagulation is recommended. The simplest risk assessment scheme is the CHADS score. In the acute setting rate control is important. Direct current cardioversion is urgently recommended for patients with AF when hemodynamic instability is present even in patients with AF and pre-excitation in Wolff-Parkinson-White syndrome. Pharmacological cardioversion may be considered in patients with AF when hemodynamic stability is present. When choosing the antiarrhythmic agent for critically ill patients only amiodarone can be considered with some exceptions due to the specific contraindications.