Acute arrhythmia is a condition covering a wide variety of rhythm disturbances. The aim of the article is to give practical recommendations for the approach and the treatment of the patient presenting with an acute arrhythmia. We discuss bradycardia and tachycardia. Tachycardias are divided into the small QRS complex tachycardias and the wide QRS complex tachycardias. Another important distinction with immediate therapeutic consequences is that between the hemodynamic stable and unstable patient. Flowcharts with diagnostic means and therapeutic schemes are added and a table with practical considerations for electrical cardioversion.