Plant tissue generally responds rapidly to sudden increases in temperature by curtailing or abolishing normal protein synthesis and producing new polypeptides known as heat shock proteins (HSP). Some of the methods used for monitoring the expression of heat shock genes are described, and the characteristics of the heat shock response in higher plants are discussed with special reference to tropical cereals. The possible role for heat shock proteins in conferring thermo-tolerance upon plant tissue is considered. The behaviour of plant tissue subjected to temperature decreases has been much less intensively studied, and varies greatly according to species and the nature of the cold treatment. No homology has yet been detected between the heat shock and the cold shock response in any plant system. Cold-induced changes in gene expression observed in a wide range of plant species are discussed with particular reference to parallel changes in cold-hardiness. Marked contrasts have been observed between the response of temperate grasses and that of tropical cereals to cold treatment, and these are discussed in relation to growth and survival at suboptimal temperatures.