This thesis investigates how teachers from different disciplines understand and implement education for sustainable development in their teaching. A nationwide questionnaire study was conducted with 3229 upper secondary school teachers representing 224 schools in Sweden. The concept of sustainable development is complex and research has shown that teachers exhibit uncertainties in their understanding of it, and that the way they conceptualize sustainable development can have consequences for how they incorporate it into their teaching. Previous research has emphasized that Education for Sustainable Development (ESD) should promote interdisciplinary and holistic learning rather than traditional subject-based learning. This is in accordance with the Swedish curriculum, which emphasizes that all teachers in all subjects should integrate education for sustainable development. The teachers in the study were grouped into four disciplines; science-, social science-, language-, and vocational/esthetical-practical teachers.  The results showed that there were many subject-bound barriers to successful implementation of  ESD. Teachers were influenced by their subject tradition in: 1) how they understand sustainable development, 2) which teaching methods they use, 3) which barriers they experience, and 4) which teaching tradition they work within. Because of these differences it is important to adjust any further training of teachers according to their discipline. This study highlights the need for improved teacher education and further training of in-service teachers; more than 70% of the teachers stated that they need such training. It also highlights the issue how strong subject-bound traditions make it difficult to implement general goals of the curricula such as sustainable development.