||Tolerance to heat stress was previously not a priority trait in tropical maize compared to tolerance to other abiotic stresses such as drought and low nitrogen. However, in recent years, heat has been identified as one of the key abiotic stresses for maize in the tropics. This is due to the fact that maize is grown year round, including during the summer and spring seasons, when it is likely to face heat stress. Also, climate change effects experienced during recent years in the tropics have resulted in increased temperatures, both maximum (Tmax) and minimum (Tmin) during key maize crop stages in the lowland tropics. This manual was developed to help maize breeders and field technicians in South and Southeast Asia, Latin America and sub-Saharan Africa to perform heat screening/ phenotyping and identify genotypic variability in tropical maize for use in breeding programs aimed at developing heat tolerant tropical maize germplasm. This manual will enable them to: Identify suitable field sites for heat stress phenotyping. Screen for heat tolerance using key secondary traits along with grain yield. Generate high quality phenotyping data by reducing experimental error through proper trial management.