Book excerpt: Soils are environments where a myriad of different organisms evolve, determining a series of functions which translate into ecosystem services that are essential for humanity. Improving our understanding of these organisms, their biodiversity and their interactions with each other, as well as with the environment, represents a major challenge. Soil ecology has its roots in natural history. The ecological approach focused on soils is notable for integrating, at least partially, the contributions of soil sciences (physics, chemistry, biochemistry). By renewing methods of observation and analysis (especially molecular ones) and through the development of experimental approaches and modeling, an ecology connected with other soil-based disciplines emerges and begins to influence aboveground ecology. Soils as a Key Component of the Critical Zone 6 presents an updated vision of knowledge and research in soil ecology as a complex system from the best French specialists.