The athlete's heart is characterized by eccentric hypertrophy of all cardiac cavities and there is a close connection to increased tone of the vagal system. As a consequence, not only arrhythmias are observed in the ECG of healthy athletes, but also changes in the QRS complex and in the ST-T-segment. Left ventricular hypertrophy is diagnosed in ECG by a positive Sokolow-Lyon index. The frequent finding of a right ventricular conduction delay is possibly due to hypertrophy of the myocardium in the apex of the right ventricle. The causes of various T wave changes are generally unclear and await further diagnostic clarification. In cases when normalization of the T-wave deviation is observed under stress, such changes are of functional nature. Echocardiography is indicated in any case to establish the heart's size and function; hypertrophic cardiomyopathy has to be excluded. Frequent cardiac dysrhythmias found in athletes are sinus bradycardia and sinus arrhythmia, less often escape rhythms are seen. A arrhythmia more often found in athletes is the respiration-dependent simple atrioventricular dissociation. Also, escape rhythms are observed in some cases with ventricular origin. Finally, a pronounced vagotonia can lead to a prolonged conduction time; AV-blocks of all degrees of severity are observed in athletes. The functional character of these arrhythmias can be easily demonstrated by their disappearance under stress.