A popular book, widely used by beginner and expert alike and generally accepted as the most useful guide to identifying British and Irish species of lichens. In addition, because of its references to air pollution tolerance and distribution, it offers the environmentalist and ecologist a concise work of reference, compact enough to be used in the field. The sixth edition has been revised to conform with the nomenclature of The Lichens of Great Britain and Ireland; Smith C.W. et al. (2009) and more recent changes. It includes a number of species that do not appear in that flora and the vast majority of entries have been updated or corrected in some way. Over 160 additional species have been added so that this edition now includes more than 1,000 species. It provides a description of most of the species likely to be found in Great Britain and Ireland, those excluded being very rare or very local.