This chapter delves into the complex subject of defining tolerance and comfort thresholds for indoor climates, with a particular focus on the challenges of establishing these thresholds during summer. It proposes a methodology grounded in climate chamber studies to set explicit bearability threshold values tailored for healthy adults. The analysis extends to the heightened heat sensitivity of demographic groups such as the elderly, individuals with pre-existing health conditions, and young children, suggesting that their threshold values be derived from excess mortality data collected during European heatwaves-an area previously underexplored in indoor climate research. Additionally, the chapter reviews the current landscape of comfort models, assessing their suitability for summer comfort evaluation. It highlights the inadequacies of the Fanger-PMV model in accurately determining optimal summer indoor air temperatures in both air-conditioned and non-air-conditioned environments. The predominance of research focusing on office buildings in Europe exposes a gap in the applicability of these findings to other building types, such as residential homes, indicating the need for broader investigatory efforts. Moreover, the limitations of empirical and adaptive models are critically examined, noting their frequent failure to account for usage nuances, such as nighttime comfort. The chapter argues that these models often fail to fully capture the diverse and dynamic ways buildings are utilized and the varied thermal comfort needs of their occupants. The methodology employed in this chapter consists of a comprehensive literature review of empirical studies concerning tolerance thresholds and comfort models. This literature review is supplemented by the analysis of published datasets, including climate chamber studies on bearability and comfort databases. Through this dual approach, the research aims to provide a robust foundation for understanding and establishing indoor climate thresholds, especially in the context of summer heat in Europe.