In view of the broad spectrum of arrhythmias and their considerable spontaneous variability, there is a need for a classification of arrhythmias as a basis for scientific and clinical decision making. From the clinical point of view, a classification should consider (a) hemodynamic consequences, (b) prognostic significance of arrhythmias, and (c) should allow assessment of efficacy of antiarrhythmic treatment. Hemodynamic consequences of tachycardias are related to the degree of heart rate: The shorter the RR interval the shorter the diastolic filling period, resulting in a decrease of the stroke volume and—above a critical heart rate—in a decrease of the cardiac output. The critical heart rate, on the other hand, is essentially related to the functional status of the heart: The more pronounced the cardiac impairment, the lower the critical heart rate. A second factor favoring hemodynamic impairment due to arrhythmias is the loss of the sequence of atrioventricular contractions. Despite the clinical relevance of hemodynamic consequences of arrhythmias, there is no accepted classification taking these aspects into account. In the past, more interest was directed toward the prognostic significance of arrhythmias. In 1971. Lown and Wolf published a classification of ventricular arrhythmias, assigning risk to advanced grades. This proposal, as those from others, took into account arrhythmias of ventricular origin only. The major concern about the Lown classification, however, relates to the consequences of maximal grading: a patient is assigned to a grade depending on the highest ranking. Thus, a person can only be in one grade, leaving all other arrhythmias grouped below as well as the true frequency of ventricular arrhythmia obscure. This implies an unacceptable loss of information. Moreover, the complex features of the Lown grading system, such as multiform (III) and consecutive forms of ventricular arrhythmia (IV) do not provide quantitative data. There is evidence, however, that quantitative analysis of complex ventricular arrhythmia is mandatory for adequate prognostic stratification. Therefore, the Lown classification has to be expanded in the sense that all groups of ventricular arrhythmia have to be evaluated quantitatively. Spontaneous variability of ventricular arrhythmia may mimic antiarrhythmic treatment. Drug efficacy, therefore, cannot be judged by the Lown classification. New statistical approaches were developed to separate spontaneous variability of ventricular arrhythmia from true-positive drug effects. High reduction rates of ventricular arrhythmia are necessary to assess this goal. Basically they have nothing to do with prognostic aspects such as risk reduction. In conclusion, there is still no classification of arrhythmias available that includes hemodynamic consequences and prognostic significance of ventricular arrhythmia and that at the same time is useful for assessment of the efficacy of antiarrhythmic treatment.