The fundamental mechanisms responsible for the production of the P, QRS, and T complexes in A-V nodal arrhythmias are reviewed. Particular emphasis is placed on the underlying mechanisms which lead to various relationship between atrial and ventricular activation in A-V noda arrhythmias. It is concluded that the classification of so-called ‘upper’, ‘middle’ and Mower’ A-V nodal beats or rhythm is only justified if one assumes antegrade and retrograde conduction times are identical. Since antegrade and retrograde conduction disturbances of varying degrees are commonly observed in the A-V junction when dealing with patients with various A-V nodal arrhythmias, the above classification of A-V nodal arrhythmias should be avoided. Atrioventricular dissociation commonly results when A-V nodal arrhythmias develop. Similarly, reciprocal beats or rhythm (or tachycardia) are not uncommonly initiated by A-V nodal arrhythmias. A-V nodal parasystole is a less common A-V nodal arrhythmia. The factors which may produce or suppress A-V nodal arrhythmias include vagal and sympathetic effects, chemical mediators, various drugs, particularly digitalis, electrolyte imbalance and various cardiac and non-cardiac diseases.