This book synthesizes the present understanding of ecosystem ecology. It is organized in four major parts. The first presents the research study methods, which are based on observation, experimentation and modelling. It is followed by an introduction to the concept of ecosystem. The various themes of present research are then tackled, including ecological hierarchies, homogeneity and heterogeneity, the role of biological diversity, and spatial and temporal scales. Finally, the work ends with a presentation of the overall functioning of the biosphere in a historical perspective.