Morphological, anatomical, and chemical studies have been made of the types of about 90% of the world's reported Phyllopsora species, of types in related genera, and of much non-type material, including specimens collected in recent years by the authors and other botanists in East Africa. The findings are reported, and on their basis an attempt is made to delimit the genus. A detailed account is given, with key, of the East African species. The following 11 species are reported for East Africa, three of them being newly described: Phyllopsora albicans Mull. Arg., P. breviuscula (Nyl.) Mull. Arg., P. buettneri (Mull. Arg.) Zahlbr., P. chlorophaea (Mull. Arg.) Zahlbr., P. confusa sp. nov., P. haemophaea (Nyl.) Mull. Arg., P. martinii sp. nov., P. mediocris sp. nov., P. pannosa Mull. Arg., P. santensis (Tuck.) comb, nov., P. thaleriza (Stirton) G. Schneider. In addition one species is reported as undetermined (Species A). The following taxa are reduced to synonymy: Phyllopsora porphyromelaena (Vainio) Zahlbr. and P. formosana Zahlbr. with P. albicans; P. brachyspora Mull. Arg. with P. breviuscula; P. subparvifolia (Mull. Arg.) Mull. Arg. and Lecidea schizophylla (Vainio) Malme with P. chlorophaea; P. kiiensis (Vainio) G. Schneider and Lecidea schizophylla var. isidiata Malme with P. haemophaea; P. miradorensis (Vainio) G. Schneider with P. santensis; Psora ochroxantha f. aelhiopica (Stizenb.) Dodge with P. thaleriza; Lecidea ernstiana Mull. Arg. with P. ochroxantha (Nyl.) Zahlbr.; P. melanocarpa Mull. Arg. with Neophyllis pachyphylla (Mull. Arg.) G. Schneider; P. schizophylloides (Malme) G. Schneider with P. munda (Malme) Zahlbr.; and P. spinulosa (Vainio) Zahlbr. with P. polydactyla (Mull. Arg.) Zahlbr. The following new names and combinations are made: Bacidia laciniosa nom. nov., B. spruceana (Mull. Arg.) comb, nov., Phyllopsora bibula (Taylor) comb, nov., P. glabriuscula (Nyl.) comb, nov., P. janeirensis (Mull. Arg.) comb, nov., P. microphyllina (Tuck, ex Nyl.) comb, nov., P. pertexta (Nyl.) comb, nov., P. pyrrhomelaena (Tuck.) comb, nov., and P. santensis (Tuck.) comb. nov.