This Booklet is planned as a forester’s guide to the recognition of the three most common and damaging decays of standing conifers in Great Britain. The fungi that cause these rots are the basidiomycetes Fomes annosus, Armillaria mellea, and Polyporus schweinilzii. They are primary root rotting organisms, infection usually beginning in the root system and developing there before proceeding into the stem, where more extensive damage is caused. In addition, however, they may all sometimes enter the stem by other means, for example through stem wounds. This Booklet includes illustrations and descriptions of both the early and the more characteristic advanced stages of rot caused by each of the fungi concerned. Brief notes on the occurrence and relative importance of each butt rot are also given.