Performing vigorous physical activity means a multiorgan system engagement, but the cardiovascular system plays a critical role. In order to provide enough oxygen to activate muscles during repeated physical activity of high intensity, the heart undergoes profound morphologic, functional and electrophysiological alterations, which have been identified as the "athlete's heart syndrome". "The athlete's heart" is a complex, but not precisely defined concept, anatomically and functionally and in relation to health and disease. It means the whole heart enlargement and/or hypertrophy of the cardiac muscle, also increasing economy of cardiac performance at rest and during physical activity with higher maximal functional capacities, all that having an adaptive response to vigorous physical activity. For morphological changes called the "athlete's heart, full-time, programed and intensive physical activity at maximal levels is primarily responsible. But as there are different kinds of physical activities, the degree of those morphological changes is highly variable. Considering many sudden cardiac deaths in sports, it is needless to say how important it is to know where the borderline is between normal changes of the heart due to physical activity and pathological changes due to some cardiac diseases. As sport has a growing socio-economic significance, sudden cardiac death events have to be reduced.