This chapter focuses on treadmill running and swimming as a means to assess endurance exercise capacity and as an exercise paradigm to elicit responses to endurance training. Starting with a founder population of heterogeneous N:National Institutes of Health rats, Koch and Britton developed a two-way artificial selection model of endurance running capacity based on maximal treadmill running performance. After 11 generations of selection, high-capacity runner and low-capacity runner selected lines differed by approximately 660 m in treadmill running endurance. J. Timothy Lightfoot et al. screened several strains of mice for endurance exercise performance using a graded treadmill test and identified DBA/2J and BALB/cJ as low- and high-performing strains. Data from animal models confirm that endurance exercise capacity and responses to training are heritable traits and these traits are determined by multiple genetic factors. The high heritability estimates for endurance exercise capacity in inbred mice suggest that a high percentage of the variation in exercise capacity is determined by genetic factors.