This paper aims at introducing the concept of emergy, originally conceived by H. T. Odum, and the main indicators it is possible to calculate using this approach. Compared to other environmental methods, Emergy Accounting assesses the environmental performance of fully natural processes and systems such as biomes, forests, and species, as well as the sustainability of human-made processes and systems on the global scale of biosphere, from a donor-side perspective, relying on the effort that is placed for the generation of goods and services by nature and processing by society. The emergy accounting method is therefore a technique of quantitative evaluation that determines the environmental value of both non-marketed and marketed resources in terms of biosphere time and space. Finally, peculiarities and main criticisms are presented. © Emilio Padilla Rosa and Jesús Ramos-Martín 2023.