Fischer (1989) summarized the detrimental effects of high temperature on wheat growth as follows: Yield reduction can occur at temperatures above a mean as low as 150C, with the spike and grain growth phases being especially sensitive; and With very hot conditions during stand establishment, lack of full ground cover will further contribute to yield loss. Mechanistically, it seems that high temperatures affect a number of physiological processes, apart from rate of development (see Discussion), although causal links between these processes and yield loss in the field environment have not previously been well established. The interaction between these mechanisms and genotype form the basis of our investigations. Probably the greatest challenge in understanding the physiological problems associated with high temperature stress is to encompass the diversity of hot environments that exist. These can be put into four broad categories: Hot dry, Hot humid, Very hot dry, and Very hot humid. Hot and very hot are climates where the mean temperature for the coolest month of the cycle is greater than 17.5 and 22.50 C, respectively. Dry and humid are climates where the mean vapor pressure deficits are above and below 10 mb, respectively for the crop cycle (Fischer and Byerlee 1991). Since the experiments described in this special report have been conducted on a multilocational basis as a collaboration between CIMMYT and national programs in warm wheat growing environments (Table 1), we anticipate that our results will be representative of the range of warm climates that exist. The International Heat Stress Genotype Experiment (IHSGE) is designed to look closely at a small number of traits which, in preliminary studies at CIMMYT and in consultation with CIMMYT outreach staff and other researchers in hot environments, seem to have potential value as predictors of yield at high temperatures.