Cardiac arrhythmias in athletes are frequent events in medical practice and require a profound diagnostic procedure. It is necessary to differentiate between harmless alterations of cardiac rhythm and potentially dangerous arrhythmias. While the former are mostly the result of an increased vagotone as a consequence of endurance training, the latter are raising the question whether intensive physical and mental strains in competitive exercise are compatible with the cardiac arrhythmias diagnosed. Vagotone-induced alterations of cardiac arrhythmias generally disappear under exercise conditions. It is essential to include the type, intensity and duration of the athletic activities into the differential-diagnostic evaluations. However, those medical considerations frequently collide with economic interests. Sinus bradycardia is a typical example of vagotone-induced arrhythmias, which may be observed especially in highly endurance-trained athletes. Sinus bradycardias are mostly asymptomatic and rarely the cause of grave complications; therapeutic interventions are only required if clinical symptoms such as orthostatic disturbances are present. The different variants of cardiac conduction defects are-within certain limits-also frequently induced by an increased vagotone; generally, they require an intensive cardiologic diagnosis. Another frequent form of arrhythmias are ventricular extrasystoles. Their dignity may be assessed by exercise ECG. Disappearance under exercise conditions is, generally, a positive sign. Diagnosis and therapy of cardiac arrhythmias are based on the established guidelines. Additionally, regular cardiologic screenings are required in high-performance athletes of all age groups.