SummaryAfter a brief historical introduction, attention is drawn to the importance of the correct sylviculturaj management of woodlands in the production of clear, disease‐free timber. Methods for the identification of the organisms which cause decay in timber are discussed, and the importance of pure culture methods is stressed. The principal contributions to the general descriptive work on timber decay in various countries are referred to, and it is pointed out how the usefulness of much of this work has been enhanced by the publication of books and monographs collating this scattered information. In a discussion of the physiology of wood‐rotting fungi the temperature relations and nutritional requirements, growth in relation to pH, and moisture content of the substratum are considered. The unsatisfactory state of present‐day knowledge of the enzymes of wood‐decaying fungi is mentioned. Recent work on the metabolic products of wood‐rotting fungi is referred to briefly. The effects of decay on the physical properties of wood are described, i.e. on the density, strength and optical properties, etc. The mechanical strength of timber, especially the toughness, may be appreciably affected in a very early stage of decay before any measurable loss in weight has occurred. The chemical changes brought about during the decomposition of wood are discussed. Rot of timber can be classified chemically into two main types: brown rots, in which only hydrolysis of cellulose and other polysaccharides occurs, and white rots, in which all the constituents of the wood, including the lignin, are attacked in varying degrees and in which oxidizing as well as hydrolysing enzymes are present. The effect of fungal attack on the microscopic structure of wood is described, and the theories put forward to explain the penetration of the cell walls by hyphae are critically examined. Methods of determining the natural resistance to decay of timber in the laboratory are referred to, and the basis of natural durability in wood is shown to lie mainly in the nature of the chemical ‘extractives’ present in the wood. Recent advances are described in the use of water‐soluble wood preservatives which become fixed in the wood by the addition of a chromate to the treating solution, and the establishment of a British Standard method of test and the use of field tests are noted. Finally, attention is drawn to the great lag in the application to practice of recent advances in the study of timber decay and its prevention.