Book excerpt: Biological soil crusts (biocrusts) are widely distributed throughout the world, and cover approximately 12% of the terrestrial surface. Biocrusts are composed of cyanobacteria, algae, lichens, mosses, and a great diversity of other microorganisms, which bind soil particles together to form a layer of biological-soil matrix on the soil surface typically of several millimetres thickness. They are important sites of regional and global microbial diversity and perform multiple ecological functions (multifunctionality). During the evolution of terrestrial life on earth, biocrusts are regarded as the main colonising photosynthetic organisms before the advent of vascular vegetation. They not only represent the early stages of terrestrial ecosystems, but also facilitate the ecosystem’s development and succession. Therefore, biocrusts are recognised as ecological engineers in the natural development of ecosystems and for the restoration of degraded terrestrial ecosystems. The development of biocrusts is highly heterogeneous, which is reflected on both temporal and spatial scales, and this heterogeneity is still clearly visible even in a small scale. However, up to now, only limited knowledge is acquired on biocrust temporal and spatial organisation. In particular there still is a large knowledge gap regarding the various biocrust communities under different developmental states and their related physiological metabolisms and ecological functions. Therefore, in-depth studies of these issues will undoubtedly further promote our understanding of the heterogeneous development of biocrusts, as well as their ecological multifunctionality in terrestrial ecosystems. The relevant contributions are expected to provide a scientific basis for the management of biocrusts and technology development (e.g. cyanobacteria-induced biocrust technology) for ecological restoration and the promotion of soil health.