Arrhythmic cardiomyopathy (ACM) is a clinical entity which can be reproduced in experimental models and which corresponds to all myocardial changes induced by chronic tachycardia. It may affect the atria and/or ventricles and, in this case, occur with all types of arrhythmia. Arrhythmia complicating a cardiomyopathy is the differential diagnosis of ventricular ACM. Nevertheless, the potential deleterious haemodynamic changes of any chronic arrhythmia may aggravate pre-existing ventricular dysfunction and, therefore, should always be considered. The development of ACM is usually progressive and depends on the heart rate, but there may also be a myocardial predisposition in certain cases. ACM is an association of haemodynamic, electrophysiological, metabolic and histological changes. Regression, which is the rule, starts in the first days following control of the ventricular rhythm but continues clinically over several months. The physiopathological mechanisms of ACM are multiple and include essentially abnormal cellular calcium concentrations. The treatment is optimally the restoration and maintenance of sinus rhythm, or at least control of the ventricular rate. Because of its curative effects, selective radiofrequency ablation of the arrhythmogenic substrate is the treatment of choice when this is localised. In chronic atrial fibrillation, when sinus rhythm cannot be maintained, the control of the ventricular response at rest and on exercise depends on pharmacological treatment, and, when ineffective, on radiofrequency modification of atrioventricular conduction with optimisation of the pacing mode.