SUMMARY The durability of eucalypts is reviewed The causes of decay and termite resistance are discussed, and the effects of factors which result in variations in decay resistance such as genetic traits, environment, growth rate and ageing of heartwood in the standing tree are considered. It is shown that decay resistance in eucalypts is due to the presence of fungitoxic extractives, and termite resistance in the species investigated is due to repellent extractives. Decay resistance is not markedly affected by growth rate per se, but is strongly associated with it in the juvenile stage, as a result of which merchantable fast—grown trees have a greater percentage of relatively non-durable wood. It is shown that in one eucalypt, deterioration in decay resistance takes place in the growing tree, this being associated with the ageing of the heartwood.