Ecosystem ecology is a branch of study and thinking within the ecological sciences that focuses on the ecosystem—a dynamic network of interactions of organisms and their environment—and the importance of these interactions to the organisms and earth system processes. The discipline represents one of two different epistemological approaches within ecology that emerged in the 20th century: a species-centric community-based approach, and a process-centric ecosystem-based approach. Both approaches study the ways in which species interact among themselves and their environment, share a common language, and share a common set of principles. The community-based approach focuses on how species’ distributions and abundances are shaped by their resource needs and tolerances to environmental conditions, and by their interactions with other species. The ecosystem-based approach represents a significant departure in that it considers both the resource needs and tolerances of species and their interactions with other species, but also factors in the contributions that species make to earth system processes (e.g., biogeochemical cycles, climate). The two perspectives are not as mutually exclusive as the phrasing of the approaches suggests, but rather offer different approaches to how we view the environment and communities and the factors that regulate them. In the 21st century, modern ecosystem science includes the influences that humans have as part of ecological communities and as drivers of change in ecological communities. This linkage within the ecosystem perspective of biology affecting the physical environment, and the recent developments that include the social and human dimensions, has positioned the approach as a critical one in understanding the relationship among global processes and the services that ecosystems provide to human well-being that is embodied in the emerging science of sustainability.