Optimal cardiac function is achieved by a coordinated pattern of cardiac contraction and relaxation that is regulated by electrical activity within the heart. Severe arrhythmias (or the development of an arrhythmia in a patient with already compromised cardiac function) may result in impaired cardiac output, increased filling pressures and increased myocardial work (haemodynamic instability). Less severe arrhythmias may not cause haemodynamic instability; however, some can be a premonitory sign of a more severe arrhythmia or sudden death (electrical instability). The following topics are discussed: Is there a treatable underlying condition?; What are the potential adverse effects of the arrhythmia?; What are the potential effects of antiarrhythmic therapy?; and Therapy for specific arrhythmias.