Abstract: A taxonomic revision is made of the agaric genus Clitocybe (Fr.) Staude in Fennoscandia (i.e., Norway, Sweden, Finland, and parts of the northwest of the U.S.S.R.), the material comprising 3220 specimens. The delimitation of the genus mainly follows that of modern authors (e.g. SINGER 1962), and in the discussion of some intergeneric differences attention is also drawn to some previously ignored ecological features. Agaricus nebularis [Batsch] Fries is acknowledged as the type species. Fairly comprehensive descriptions are given of the infrageneric diagnostic characters. Among the characters whose genetically controlled variation and diagnostic importance has often been ignored or undervalued by previous authors are: certain features of the pileus surface, the organization of the basal mycelium of the stipe, hygrophany, odour, the responses of some parts of the dried fruit body under ultra-violet light, the colour of the spore deposit, several other spore characters, some of which are taken as the basis for a new system of spore shape classification, and pigmentation. Some characters are supposed to be original, and others derived. Several macrochemical reagents were tested, of which KOH proyed to be the most useful. Information on the cytology (e.g. the chromosome number) of one species is given. Some macroscopical, sporal, anatomical, ecological and other characters as well as certain responses to ultra-violet light are shown to be ± uniform throughout Clitocybe, and may thus be diagnostic of the genus. Considerable attention is paid to the habitat (substrate, forest site type, relation to lime content of ground, relation to human influence), fruiting time, and geographical distribution of the species, which may be grouped in various, often clear-cut, categories with regard to each of these aspects. The largest numbers of species are found in the coniferous and mixed deciduous and coniferous, hemiboreal and southern boreal forests, the number of eastern taxa being considerable. Some species occur in alpine Dryas heaths. Three species are recorded for the first time outside N. America, and two outside Asia. 3 subgenera, 16 sections, 2 subsections, 43 species, and 2 varieties are recognized, of which 1, 6 (in addition, 2 provisionally), 2, 14, and 1, respectively, are described as new. 4 new combinations are introduced. Lectotypes or neotypes are proposed for several species. The author citations of some sectional and specific names are corrected.