The lichen flora of the county of Gwynedd, north-west Wales, is described. The flora includes 851 species and 19 subspecies and varieties. The work comprises sections on: climate; geology; land use and vegetation; history of lichenology, 1680–1985; corticolous flora, with classification and description of the important woodland sites; inland saxicolous flora, dealing with acid, intermediate and basic rock, the effects of altitude and aspect, the flora of seepages, streams and lakes; heath and moorland flora; coastal flora classified into the recognized zones of the marine littoral, mesic, submesic and xeric supralittoral; aspects of conservation, pollution; and the lichen flora itself. The latter includes a brief description of the ecology and distribution of each species and all of the currently known 10-km2 records for the species in the county.