Members of the genus Plagiostoma inhabit leaves, stems, twigs, and branches of woody and herbaceous plants predominantly in the temperate Northern Hemisphere. An account of all known species of Plagiostoma including Cryptodiaporthe is presented based on analyses of morphological, cultural, and DNA sequence data. Multigene phylogenetic analyses of DNA sequences from four genes ([beta]-tubulin, ITS, rpb2, and tef1-[alpha]) revealed eight previously undescribed phylogenetic species and an association between a clade composed of 11 species of Plagiostoma and the host family Salicaceae. In this paper these eight new species of Plagiostoma are described, four species are redescribed, and four new combinations are proposed. A key to the 25 accepted species of Plagiostoma based on host, shape, and size of perithecia, perithecial arrangement in the host, and microscopic characteristics of the asci and ascospores is provided. Disposition of additional names in Cryptodiaporthe and Plagiostoma is also discussed. Key words: Ascomycota, Betulaceae, epitypification, Fraxinus, new species, phylogeny, Salicaceae, Sordariomycetidae. Taxonomic novelties: Plagiostoma dilatatum L.C. Mejia, sp. nov., Plagiostoma exstocollum L.C. Mejia, sp. nov., Plagiostoma imperceptibile L.C. Mejia, sp. nov., Plagiostoma oregonense L.C. Mejia, sp. nov., Plagiostoma ovalisporum L.C. Mejia, sp. nov., Plagiostoma samuelsii L.C. Mejia, sp. nov., Plagiostoma versatile L.C. Mejia & Sogonov, sp. nov., Plagiostoma yunnanense L.C. Mejia & Zhu L. Yang, sp. nov., Plagiostoma apiculatum (Wallr.) L.C. Mejia, comb. nov., Plagiostoma convexum (Preuss) L.C. Mejia, comb. nov., Plagiostomapopulinum (Fuckel) L.C. Mejia, comb. nov., Plagiostomapulchellum (Sacc. & Briard) L.C. Mejia, comb. nov.