This study highlights the barriers that secondary teachers in Greece face dealing with the implementation of Environmental Education programs throughout a period of more than 30 years in the Greek educational system. Participants were 20 secondary teachers, emanating from various disciplines. Seven were "senior" teachers, having implemented EE projects before its official institutionalization, in 1990, and 13 were "junior", meaning that they implemented EE projects during the Decade of Education for Sustainable Development (2005-2014). In-depth, face-to-face, semi-structured interviews were used to identify teachers' views about the barriers they faced regarding changes or transformations in the realization of EE during the periods under focus. Collected data were subjected to grounded theory analysis and revealed that there were similarities and differences concerning the barriers encountered in the periods of study. Barriers are of structural, financial and economic nature as well as pedagogical and epistemological. These refer to the position of EE into the secondary schools and to the orientation of the school within the general educational system. Results indicate rather a restriction of EE into schools instead of expansion and flourishment following the international progress made, in theoretical and practical level, and the experience that have been gained worldwide after decades of practicing.