The chapter discusses the development and application of a standard predictive index of human response to thermal environments. It introduces three rational indices: ASHRAE's Standard Effective Temperature (SET*), Fanger's Predicted Mean Vote (PMV), and Winslow's Skin Wettedness Index of Thermal Discomfort (DISC). These indices are based on the energy exchange between the skin and the environment, the body's regulation of internal temperature, and the physical properties of the skin surface. The chapter outlines the human heat balance equation and its significance, explaining how it describes the heat exchange at the skin surface. It also discusses the effective temperature (ET*) concept, which is defined as the dry bulb temperature of an isothermal environment at 50% relative humidity (RH) that results in the same skin wettedness and heat exchange as the actual test environment. The chapter proposes a new index, PMV*, which combines the effects of heat load and thermo-regulatory strain, making it more responsive to changes in humidity and clothing vapor permeability. The PMV* index is illustrated through examples and a comfort-humidity chart, demonstrating its ability to predict thermal sensation and comfort in various environments. The chapter concludes by discussing the advantages of PMV* over the original PMV index and its potential applications in HVAC engineering and occupational safety.The chapter discusses the development and application of a standard predictive index of human response to thermal environments. It introduces three rational indices: ASHRAE's Standard Effective Temperature (SET*), Fanger's Predicted Mean Vote (PMV), and Winslow's Skin Wettedness Index of Thermal Discomfort (DISC). These indices are based on the energy exchange between the skin and the environment, the body's regulation of internal temperature, and the physical properties of the skin surface. The chapter outlines the human heat balance equation and its significance, explaining how it describes the heat exchange at the skin surface. It also discusses the effective temperature (ET*) concept, which is defined as the dry bulb temperature of an isothermal environment at 50% relative humidity (RH) that results in the same skin wettedness and heat exchange as the actual test environment. The chapter proposes a new index, PMV*, which combines the effects of heat load and thermo-regulatory strain, making it more responsive to changes in humidity and clothing vapor permeability. The PMV* index is illustrated through examples and a comfort-humidity chart, demonstrating its ability to predict thermal sensation and comfort in various environments. The chapter concludes by discussing the advantages of PMV* over the original PMV index and its potential applications in HVAC engineering and occupational safety.