Masters athletes are typically older than 35 years of age and systematically train for, and compete in, organized forms of sport specifically designed for older adults. They are motivated to participate in masters sport for a wide variety of reasons. Age-related declines in endurance performance are observed across the endurance sports of running, orienteering, rowing, and swimming. These declines are curvilinear from age 35 years until approximately age 60–70 years and exponential thereafter. The decline in endurance performance appears primarily due to an age-related decrease in VO_2max secondary to an age-related decrease in HR_max and possible age-related declines in stroke volume and arteriovenous oxygen difference. While performance velocity at lactate threshold decreases with age in masters endurance athletes, it appears to increase relative to VO_2max while exercise economy is maintained. There also appears an age-related decrease in active muscle mass, type II muscle fiber size, and blood volume that contribute to decreased endurance performance. However, research suggests that maintenance of training intensity and volume into older age may mediate the rate of age-related decline in VO_2max, stroke volume, arteriovenous oxygen difference, blood volume, and muscle mass in masters endurance athletes.