Models for outdoor thermal comfort are essential for guiding urban design and promoting the effective use of open spaces. Since thermal equilibrium alone cannot fully explain human comfort outdoors, it is necessary to incorporate both rational heat balance models and adaptive mechanisms. This chapter develops adaptive-rational models that integrate two established indices—Physiological Equivalent Temperature (PET) and Universal Thermal Climate Index (UTCI)—with adaptive and extension strategies. Field validation demonstrates that the adaptive-rational approach substantially improves predictive accuracy and robustness compared to conventional rational models. Specifically, PET-based adaptive-rational models achieve accuracy gains of up to 83% and robustness improvements of 86%, while UTCI-based adaptive-rational models reach improvements of 87%. These results highlight the effectiveness of combining adaptive principles with rational formulations, providing a more reliable framework for evaluating outdoor thermal comfort and supporting the design of thermally livable urban environments.