This contribution considers all taxa included in the genus Psora by ZAHLBRUCKNER in his Catalogus Lichenum Universalis as well as those subsequently newly described or transferred to the genus. The general part gives a comparative account of the features of the morphology and anatomy of the species, and assesses the importance of these characters in the basic systematica of the genus. In the structure of the thallus the structure of the cortex arrangement of the algae and the presence and form of under cortex and of the rhizines have been found to be important taxonomic characters to the separation of the genera. Likewise, in the structure of the apothecia, the most important taxonomic features are their formation and size, structure of the excipulum and the form of the ascus, including the amyloid reaction of the tholus, the paraphyses and the form and size of the spores. The lichen substances of nearly all the species have been analysed by reactions of K, C, KC, PD and J and by thin layer chromatography. The lichen substances of several species have been identified or unidentified substances have been proved. The results of this investigation are included in the respective descriptions of each species. Altogether, about 350 collections were investigated. Where possible the type of each taxon was also studied. Varieties and forms have not been designated except for Psora decipiens var. galactina. The original genus Psora, as understood by previous authors is here emended to include only 17 species (see synopsis). 4 genera are newly described in the present account, and the number of species in each indicated in parentheses: Psorinia (2), Psorula ( 2 ) , Trapeliopsis (5) and Xanthopsora (1 ) . The following taxa, part of Psora s.l., havo been assigned to the following already existing genera; to Astroplaca (1), Chrysopsora (1), Eremastrella (1), Hypocenomyce (3) and to Lecidoma (1). Several species must be referred to genera in no way related to the Psora s.l. These are Lobiona (1), Neophyllis (2), Phyllopsora (19) and Schaereria (1). 19 species cannot at present be referred to any genus, and they have been included here under species incertae sedis. 33 species in all are excluded. It has not been possible to examine material, either type or otherwise, of 14 other species. 27 previously described taxa have been reduced to synonymy (see list p. 45). 15 new combinations are made (see list p. 270). Lectotypes have been selected for four species: Psora russellii, Lecidea glauca, L, infirmata and L. ventosa. A key is included for all taxa with a psoroid growth habit. Instead of a key of genera a comparative table (p. l6l) incorporating the diagnostic features of the 10 genera comprising Psora s.l. is provided