Biocrusts are photosynthetic communities that extensively cover Earth's exposed soil surfaces, occupying the critical interface between the atmosphere and the pedosphere. Here, we present a comprehensive review of the definition, distribution, composition, structure, and function of biocrusts, emphasizing their relevance to the Earth sciences through influences on pedogenesis, landscape evolution, hydrological processes, surface energy balance, and biogeochemical cycles, both in the present and throughout Earth's history. We summarize both the established understanding and existing knowledge gaps regarding the complex interactions between biocrusts and the Earth system. Through this review, we demonstrate that biocrusts are a crucial linkage among various subdisciplines of Earth sciences and adjacent fields, thereby contributing to the refinement and advancement of Earth sciences to keep pace with global changes. As a lever to support vital Earth system processes, biocrust research helps find transdisciplinary solutions to combat climate change, conserve biodiversity, and promote sustainable land use. Biocrusts are photosynthetic communities inhabiting the interface between atmosphere and pedosphere, covering about 12% of Earth's terrestrial surface. Biocrusts influence the Earth system by mediating pedogenesis, landscape evolution, surface energy balance, and hydrological and biogeochemical cycles. Biocrusts form a link among geology, geomorphology, climatology, hydrology, soil science, environmental science, cryosphere science, and geospatial science. Biocrust research provides potential solutions to combat climate change, conserve biodiversity, and promote sustainable land use.