The physiology of heat stress and heat resistance has been reviewed rather extensively. Damage caused by high stress temperatures as a result of physiological disorders is common in crop plants. Such extreme damage is associated with death of cells, tissues, organs, or whole plants. Genetic resources for heat tolerance are apparently quite common within the normal breeding germplasm of a number of crop species, as the case is for rice, potato, soybean, tomato, and other crops. On the other hand, for a good number of crop plants, the extent of existing genetic variation in heat tolerance has hardly been explored. The simplest approach to selection for heat tolerance would be to utilize the natural-stress field environment. The efficiency of the field environment in affecting progress in selection depends on the stability of this environment and the nature of the required heat tolerance, the nature of the required heat tolerance.