This publication presents a classification of marine habitats for the shores and seabed around Britain and Ireland. It has been substantially revised and updated from the previous version which was developed by JNCC’s Marine Nature Conservation Review (MNCR) as part of the EC Life Naturefunded BioMar programme (Connor et al. 1997a, b). This new version reflects the continuing needs of end-users for improved information about each habitat, which is presented in a variety of interactive ways in this web-based version. As a result of re-analysis of field data, including new data from recent surveys in candidate Special Areas of Conservation (cSACs), a number of new habitat types have been described. The classification has been restructured, both to reflect improved understanding of the inter-relationship of habitat types and to align itself more closely with the European EUNIS habitat classification system (http://mrw.wallonie.be/dgrne/sibw/EUNIS/home.html).