The correct diagnosis of complex arrhythmias requires an exact knowledge of arrhythmia mechanisms and their application in each specific case. In a patient with dilated cardiomyopathy and nonspecific sinus tachycardia, the interpretation of the coronary vein sinus electrogram could quickly raise the possibility of left atrial tachycardia. In a patient with tachycardia, pre-existing left bundle branch block, and syncope, the induction mechanism of the tachycardia in the electrophysiological examination as well as in the detailed analysis of the His bundle electrogram were finally identified as an unusual presentation of AV nodal re-entrant tachycardia. In the patient with broad complex tachycardia and coronary heart disease, successful ablation was only possible with the knowledge that the access to the area of the slow pathway is only possible via the epicardia. Atrial macro re-entrant tachycardia after atrial fibrillation ablation is a growing challenge in clinical rhythmology. An example of this type could be analyzed with the case history of left atrial perimitral atrial flutter after pulmonary vein ablation.