Older ("Masters') athletes strive to maintain or even improve upon the performance they achieved at younger ages, but declines in athletic performance are inevitable with aging. In this review, we describe changes in peak endurance exercise performance with advancing age as well as physiological factors responsible for those changes. Peak endurance performance is maintained until ~35 years of age, followed by modest decreases until 50-60 years of age, with progressively steeper declines thereafter. Among the 3 main physiological determinants of endurance exercise performance (i.e., maximal oxygen consumption, VO2max; lactate threshold; and exercise economy), a progressive reduction in VO2max appears to be the primary mechanism associated with declines in endurance performance with age. A reduction in lactate threshold, i.e., the exercise intensity at which blood lactate concentration increases significantly above baseline, also contributes to the reduction in endurance performance with aging, although this may be secondary to decreases in VO2max. In contrast, exercise economy (i.e., metabolic cost of sustained submaximal exercise) does not change with age in endurance-trained adults. Decreases in maximal stroke volume, heart rate, and arterio-venous O2 difference all appear to contribute to the age-related reductions in VO2max in endurance-trained athletes. Declines in endurance exercise performance and its physiological determinants with aging appear to be mediated in large part by a reduction in the intensity (velocity) and volume of the exercise that can be performed during training sessions. Given their impressive peak performance capability and physiological function capacity, Masters athletes remain a fascinating model of "exceptionally successful aging" and, therefore, are highly deserving of our continued scientific attention as physiologists.