Timber exposed to moisture for any length of time is at risk from attack by wood-rotting fungi. To obtain a good service life, it is important to select a timber species that is either naturally durable or amenable to preservative treatment. This Digest explains the classification of durability and treatability for timber, and classifies these properties for over 150 species. It is concerned with natural durability only in relation to fungal decay and not to resistance to attack by insects and marine borers. This Digest replaces Digest 296 which is withdrawn. 8 pages.