This taxonomic study of the hyphomycete genus Drechslera is based on type specimens, and on fresh collections and herbarium specimens principally from North America. Characters of conidia and conidiophores, and the growth characteristics of the species in artificial culture, are described and illustrated. Two keys to species are included: one based on characters of conidia and conidiophores; the second based on growth characteristics on sucrose proline agar. Fifteen species, one composed of two formae speciales, are accepted. Their names and some of the well-known synonyms are: D. avenacea (Curtis ex Cooke) Shoem. (= Helminthosporium, avenae Eidam); D. bromi (Died.) Shoem.; D. catenaria (Drechs.) Ito; D. dactylidis n. sp.; D. dictyoides (Drechs.) Shoem. f. sp. dictyoides; D. dictyoides f. sp. perenne (Braverman & Graham) n. comb.; D. erythrospila (Drechs.) Shoem.; D. fugax (Wallr.) Shoem. in Hughes (= H. stenacrum Drechs.); D. gigantea (Heald & Wolf) Ito; D. graminea (Rab. ex Schlecht.) Shoem.; D. phlei (Graham) Shoem.; D. poae (Baudys) n. comb. (= H. vagans Drechs.); D. siccans (Drechs.) Shoem.; D. teres (Sacc.) Shoem.; D. tritici-repentis (Died.) Shoem. (= Drechslera tritici-vulgaris (Nisikado) Ito); and D. tuberosa (Atk.) Shoem. (= H. secalis Whitehead & Dickson, = H. japonicum Ito & Kurib., = H. fragosoi Bubak). Also included are the perfect states of six species: Pyrenophora chaetomioides Speg. (st. conid. D. avenacea); P. bromi (Died.) Drechs. (st. conid. D. bromi); P. teres Drechs. (st. conid. D. teres); P. tritici-repentis (Died.) Drechs. (st. conid. D. tritici-repentis); P. japonica Ito & Kurib. in Ito (st. conid. D. tuberosa), and the perfect state of D. dactylidis.