The risk of biodeterioration of timber is discussed under the headings of wood structure, environmental factors (e.g. Supply of oxygen, water and nutrients) and organisms causing deterioration (including bacteria, molds and blue-stain fungi, soft-rot fungi, wood-destroying Basidiomycetes and the insect species <1Anobium punctatum, Xestobium rufovillosum and Hylotrupe bajulus). Stresses the need for careful analysis of the risk of biological attack and for balancing costs of improved design against those of preservative treatment. The problem of condensation in many constructions, especially roofs, is particularly noted.