Abstract: Biological soil crusts are built up by cyanobacteria, green algae, soil lichens, mosses and fungi. They are important communities in arid and semi-arid ecosystems and influence the ecosystem mosaic and processes. In addition to the influence on the hydrological conditions, the biological crust also stabilises the topsoil, reduces soil erosion, and enhances the nitrogen pools by nitrogen fixation. The crusts are important for the rehabilitation of desertified arid and semi-arid lands and provide a natural protective soil surface cover.