In this episode, we delved into various aspects of endurance training physiology. Stephen Seiler, one of the world’s leading sports physiologists, provided invaluable insights into these topics. We explored the significance of training volume and intensity in optimizing athletic performance, the definition of thresholds and the distinction between internal and external load in training. Seiler underscored the importance of monitoring both internal factors (such as heart rate, perceived exertion) and external factors (such as power) to gauge training effectiveness accurately. A significant portion of the discussion centered on the psychological limitations of Zone 2 training. Overall, this chapter provided a comprehensive exploration of key concepts in endurance exercise physiology, offering valuable perspectives on optimizing training strategies for athletes striving to reach their full potential.