Biological soil crusts (BSCs) were reported from six out of seven biomes along the BIOTA Southern Africa transects. Seven morphological BSC types were distinguished: three cyanobacteria-dominated crusts (types 1and 2), one with additional cyanolichens (type 3), one type with chlorolichens (4), and one with bryophytes-either liverworts (type 5a) or mosses (type 5b). The hypolithic crust type (6) was restricted to quartz gravel pavements, and another one comprised the unique lichen fi elds of the Namib Desert (type 7). In total, at least 120 different species with 58 cyanobacteria, 29 green algae, one heterokontalgae, 12 cyanolichens, 14 chlorolichens, two genera of liverworts, and three genera of mosses were found, placing these BSCs among the most diverse worldwide. They contribute considerably to the biodiversity of the arid and semi-arid regions of south-western Africa. The taxonomic diversity of cyanobacteria was higher in the winter rainfall zone than in the summer rainfall zone (54 versus 32 species). Soil photosynthetic biomass, carbon content and the number of BSCs revealed the same significant distribution patterns. Rainfall frequency and duration of dry periods rather than the precipitation amount seem to be the main factors infl uencing BSC growth and succession. This article is mostly based on the publication of Büdel et al. (2009).