It is necessary to provide an environmental dimension to all educational practices if we really want to make progress towards education for sustainability. The curricula should be reoriented based on the principles and focus of education for sustainability. This paper presents a research process carried out by a working group formed by teachers from the Catalan Green School Programme, by representatives from the Department of the Environment and Housing and the Department of Education, the Catalan Society for Environmental Education and the GRECA (Research Group of Scientific and Environmental Education, University of Girona) as a coordinator. The objectives of the research were to: (i) define the concept of curriculum greening within compulsory education; (ii) characterize curriculum greening at the early, primary and secondary education levels. (iii) Identify strategies for curriculum greening in schools. The research is situated within the theoretical-critical educational research paradigm and follows a process of participatory action research. The analysis is fundamentally qualitative. The definition and the characterization of the curriculum greening are presented, as well as a proposal of strategies for greening the curriculum in compulsory education.