Natural resistance of wood from differnt tree species to microbial deterioration differs considerably. Individual trees of the same species may also vary a great deal in decay resistance. Natural durability varies within the same tree too, especially in species with very durable heartwood. In the durable species, decay resistance decreases incessantly from the outer heartwood to the pith. Toxic extractive substances deposited during the formation of the heartwood are the principal source of decay resistance. Most of the heartwood extractives known to inhibit decay are phenols. Carvacrol, 2-thujaplicin, quercetin pinosyluin, and tannins all seem to contribute to decay resistance, other mechanisms of natural durability of wood, such as the lignification of wood cell walls, crystallinity of the cellulose, depletion of reserve food materials, higher moisture rcquirments for wood-inhabiting fungi, and different resistance of the wood structure to white-rot and brown-rot fungi arc also discussed, in this paper.