A taxonomic revision is presented for the lichen genus Micarea in Europe, with special emphasis on those species occurring in the British Isles. In brief, this genus is here circumscribed to include most crustose lichens with lecideine (biatorine) apothecia, a poorly defined excipulum (sometimes absent altogether) of radiating paraphysis-like hyphae, a non-granular epithecium, Lecanora-type, 8-spored asci, simple to multiseptate, hyaline ascospores, and a 'grass-green' phycobiont usually of the so-called 'micareoid' type. Comparisons and possible relationships with similar genera are discussed. Noteworthy discoveries made during this study include the finding of cephalodia in three species, dimorphic paraphyses in several species, and a wide array of anamorphic forms, with three species each found to have three conidial states. Type studies have been made for nearly all names known, as well as those suspected, to be referable to Micarea in its present, wider concept. Forty-five species are recognised, of which 31 are confirmed from Britain. Seven species are new to science: Micarea adnata, M. curvata, M. hedlundii, M. muhrii, M. nigella, M. olivacea, and M. myriocarpa Vězda & V. Wirth ex Coppins. Several described species are included in Micarea for the first time, and additional name changes are required for nomenclatural reasons; nine new combinations result: M. assimilata (Nyl.), M. crassipes (Th.Fr.), M. elachista (Körber) Coppins & R. Sant., M. globulosella (Nyl.), M. intrusa (Th.Fr.) Coppins & Kilias, M. lignaria var. endoleuca (Leighton) , M. melaenida (Nyl.), M. melanobola (Nyl.), and M. subviolascens (Magnusson). Several taxa are excluded from the genus and the new combinations, Psilolechia clavulifera (Nyl.) and Bacidia prasinata (Tuck.), are proposed. Keys for the identification of all the accepted European taxa are given. The taxonomic parts are preceded by an outline of the historical background to the study of Micarea, and details of materials and methods employed in this study. Detailed accounts of the morphology, chemistry, and ecology in the genus are provided, and a discussion of distributions is supported by maps for the British taxa. All Micarea species occur on acidic, nutrient poor substrata, and most are confined to cool-temperate, boreal, or oceanic regions; a few occur in arctic-alpine areas but the genus is poorly represented in dry, lowland, Mediterranean regions. Prior to this study, most of the species were little-known or misunderstood; clarification of their taxonomy has been achieved by paying particular attention to their anamorphic states, chemistry (including pigmentation), and detailed anatomy. Consideration of the distribution and ecology of the species has proved invaluable in ordering the taxonomic chaos which previously surrounded the notoriously variable species of the genus.