Our understanding of thermal stress in cotton had been relegated to extremes of cold (below 15 C) or heat (40 C, heat shock temperatures) until the identification of the thermal kinetic window of optimal enzyme function. The reported thermal kinetic window for cotton determined from enzyme analyses is between 23.5 C and 32 C. This finding has been supported by recent studies of the thermal response of pigment protein synthesis and chlorophyll fluorescence analyses. This report presents a brief review of the development of the thermal kinetic window concept and some recent findings of leaf responses to the daily changes in temperature.