Presents a comparative discussion of soil crusts on a world-wide scale. In arid lands, where vegetation is sparse or absent, the open ground is generally covered by a community of small, highly specialized organisms. Cyanabacteria, algae, microfungi, lichens, and bryophytes aggregate soil particles to form a coherent skin - the biological soil crust. This book offers a synthesis of the biology of soil crusts and their importance as an ecosystem component. Composition and functioning of different soil-crust types are discussed and case studies are used to reflect the impact of crusts on landscape hydrology, soil stability, nutrient cycles and land management.