Degree days are commonly used by industry and researchers to estimate expected crop development. This assumes that cotton's potential development is largely a function of temperature. Controlled environment studies show that the function currently used to calculate Degree Days does not accurately reflect the effect of high temperatures on development. Preliminary analysis of the data shows that the inclusion of an optimum temperature in the degree day function in addition to the base temperature of 12'C can make more consistent predictions of cotton development. Improving this function will enable better predictions of cotton development in a greater range of environments and seasons.