This review article discusses how climate change affects the thermal tolerance of ectothermic animals, focusing on oxygen limitation as a key physiological constraint. It highlights that the thermal tolerance of metazoa is limited by complex systemic processes, particularly the aerobic scope, which is the capacity for aerobic activity. At temperatures outside the thermal optimum, aerobic scope decreases, leading to anaerobic metabolism and the activation of heat shock proteins and antioxidant defenses. Oxygen levels in body fluids may decrease due to increased oxygen demand at high temperatures or insufficient aerobic capacity at low temperatures. The adjustment of mitochondrial densities is crucial for maintaining aerobic scope and shifting thermal tolerance. The paper also explores the relationship between oxygen limitation and thermal tolerance, noting that oxygen delivery systems define thermal tolerance limits before molecular functions are disturbed. The study emphasizes that thermal tolerance is influenced by the complexity of physiological functions, with oxygen limitation playing a central role. It discusses how thermal tolerance windows vary between species and populations, and how adaptation to temperature changes involves shifts in tolerance thresholds. The paper also addresses the implications of these findings for ecological and evolutionary processes, highlighting the importance of oxygen availability in determining the limits of thermal tolerance in both aquatic and terrestrial ectotherms. The review concludes that oxygen limitation is a critical factor in thermal tolerance, with implications for the survival and distribution of ectothermic species in changing climates.This review article discusses how climate change affects the thermal tolerance of ectothermic animals, focusing on oxygen limitation as a key physiological constraint. It highlights that the thermal tolerance of metazoa is limited by complex systemic processes, particularly the aerobic scope, which is the capacity for aerobic activity. At temperatures outside the thermal optimum, aerobic scope decreases, leading to anaerobic metabolism and the activation of heat shock proteins and antioxidant defenses. Oxygen levels in body fluids may decrease due to increased oxygen demand at high temperatures or insufficient aerobic capacity at low temperatures. The adjustment of mitochondrial densities is crucial for maintaining aerobic scope and shifting thermal tolerance. The paper also explores the relationship between oxygen limitation and thermal tolerance, noting that oxygen delivery systems define thermal tolerance limits before molecular functions are disturbed. The study emphasizes that thermal tolerance is influenced by the complexity of physiological functions, with oxygen limitation playing a central role. It discusses how thermal tolerance windows vary between species and populations, and how adaptation to temperature changes involves shifts in tolerance thresholds. The paper also addresses the implications of these findings for ecological and evolutionary processes, highlighting the importance of oxygen availability in determining the limits of thermal tolerance in both aquatic and terrestrial ectotherms. The review concludes that oxygen limitation is a critical factor in thermal tolerance, with implications for the survival and distribution of ectothermic species in changing climates.