The two wood-rotting basidiomycetes Armillaria mellea and Fomes annosus are chosen to illustrate observations and experiments on resistance phenomena. The degree of suppression and position in a stand affect resistance of trees to these root parasites, competition for light and water being important factors. Resistance to killing tends to increase with age and often leads to stabilization of disease gaps. Different species of a genus may vary in resistance to root disease, but more spectacular differences often occur between genera. This may lead to striking contrasts in the incidence of killing, or the type of attack may be fundamentally different, as with species susceptible to butt-rot. The bark is an important barrier to infection and when it is damaged the root may be invaded by wound-colonizers. Under some conditions secondary phellogens seal off infected tissues and growth of new roots may promote recovery. Production of resin is an important defence mechanism, particularly in pines, and is greatly affected by the moisture regime in the tree. Some volatile constituents of resin strongly inhibit fungi. Toxic substances, generally phenols, are often formed in developing heartwood and in some species confer marked resistance to decay. Pinosylvins are present in the heartwood of pines and are also produced in response to fungal infection of the sapwood. An unusual situation is found in Abies grandis: permeability to gaseous diffusion is very low in the naturally waterlogged heartwood and growth of F. annosus is apparently limited by lack of oxygen.