Comfortable and healthy outdoor microclimates are essential for advancing sustainable urban development. This chapter presents a comparative evaluation of widely applied outdoor thermal comfort indices, including Predicted Mean Vote (PMV), Wet-Bulb Globe Temperature (WBGT), Physiological Equivalent Temperature (PET), Standard Effective Temperature (SET*), and the Universal Thermal Climate Index (UTCI). Each index differs in its energy balance assumptions, mathematical structure, and applicable boundary conditions, which affects their performance in real-world environments. Field surveys and on-site meteorological measurements conducted at a university campus in Guangzhou, China, were used to assess the indices under hot summer conditions. Results indicate strong correlations between operative temperature and thermal indices such as WBGT, PET, SET*, UTCI, Tmrt, and PMV. However, when operative temperatures exceeded 34 °C, their predictive relationships with mean thermal sensation vote (MTSV) became less consistent. These findings highlight both the value and the limitations of current indices, and underscore the need for refined classification schemes to better address outdoor heat stress in warm and humid climates.