Reliable classification and characterisation of the natural durability of timber species, and the effectiveness of preservative treatments, take several decades. They are essential, though, to provide a basis for robust specification of timber for use in construction and for satisfactory service life. This paper reports recent findings from a programme of long term testing at BRE to benchmark the performance of timber in ground contact. It concludes that most of the new hardwood species becoming available in the UK market are, in fact, of limited durability. It recognises, though, that treatment can extend the service life of many softwood species, and also concludes that a significant improvement in the service life of treated spruce in ground contact situations can be achieved by incising prior to treatment.This paper complements IP 6/99, IP 2/01 and Digest 429. 8 pages.