Although complexity surrounds us, its inherent uncertainty, ambiguity, and contradiction can at first make complex systems appear inscrutable. Ecosystems, for instance, are nonlinear, self-organizing, seemingly chaotic structures in which individuals interact both with each other and with the myriad biotic and abiotic components of their surroundings across geographies as well as spatial and temporal scales. In the face of such complexity, ecologists have long sought tools to streamline and aggregate information. Among them, in the 1980s, T. F. H. Allen and Thomas B. Starr implemented a burgeoning concept from business administration: hierarchy theory. Cutting-edge when Hierarchy was first published, their approach to unraveling complexity is now integrated into mainstream ecological thought. This thoroughly revised and expanded second edition of Hierarchy reflects the assimilation of hierarchy theory into ecological research, its successful application to the understanding of complex systems, and the many developments in thought since. Because hierarchies and levels are habitual parts of human thinking, hierarchy theory has proven to be the most intuitive and tractable vehicle for addressing complexity. By allowing researchers to look explicitly at only the entities and interconnections that are relevant to a specific research question, hierarchically informed data analysis has enabled a revolution in ecological understanding. With this new edition of Hierarchy, that revolution continues.