The key for the diagnosis of atrial fibrillation is the complete irregularity of the R-R intervals in the absence of P waves. In atrial flutter, the analysis of the flutter waves itself is important. In doing so, common and uncommon flutter can be distinguished. The A-V conduction and therefore the R-R intervals can be constant or irregular. If widened QRS complexes are registered intermittently, aberrant conduction has to be differentiated from ventricular premature beats. In this context die analysis of the width and the shape of the QRS complexes as well as the R-R intervals preceding and following the abnormal beats are important criteria. In principle, this holds true as well for the diagnosis of WPW syndrome in the presence of AF. If the surface ECG does not allow a clear-cut diagnosis, the indication for invasive electrophysiological testing may be given in a few cases.