While the benefits of physical exercise for human health are well known, their molecular mechanisms are not completely understood. We have studied the effects of exercise on cellular senescence and telomere biology. Telomeres are protective molecular caps at the ends of the chromosomes and are vital for cellular function and aging. We performed animal studies on mice using the voluntary running wheel model as well as cross-sectional studies and recently a randomized, controlled, prospective study comparing 3 x 45min aerobic endurance, intensive interval and strength training for 6 months. The main finding: Telomerase activity and blood cell telomere-length were increased by both modalities of endurance training, but not in the control group or the strength training group. The remarkable effects of endurance training on telomere parameters represent a novel molecular mechanism of physical activity and underscore its importance for healthy cardiovascular aging and disease prevention.