Ecological Site Classification (ESC) will help forest managers to select tree species, and to make related decisions based on an appreciation of the ecological potential of sites. The classification focuses on the key factors of site that influence tree growth, and that are important to the rest of the ecosystem. This site-orientated approach to tree species selection will assist users to practise sustainable forestry. For example, by selecting species suitable to a site it w ill discourage the approach of selecting a species and then altering site conditions by excessive ground preparation and fertilizer applications. The multi-dimensional approach to site classification, assessing four climate and two soil factors, is similar to that adopted in the Biogeoclimatic Ecosystem Classification (BEC) of British Columbia. However, unlike BEC, it is applicable to all kinds of woodlands, from plantations of a single species through to semi-natural woodlands, as well as to many kinds of non-wooded land. The close link between ESC and the National Vegetation Classification (NVC) provides clear evidence of the ecological requirements of different vegetation communities on a given site. This Bulletin contains a full description of the methodology behind ESC, and provides an explanatory foundation for users of the software ESC-DSS. It is recommended reading for forest managers, woodland owners, academics, students and others concerned with the ecological potential of site types in Britain.