This article describes the results of a nationwide questionnaire study of 3229 Swedish upper secondary school teachers' conceptual understanding of sustainable development in relation to their subject discipline and teaching experience. Previous research has shown that teachers have difficulties understanding the complex concept of sustainable development. According to the Swedish curriculum, all teachers in all subjects should integrate a holistic perspective of sustainable development including economic, ecological, and social dimensions. This study shows that teachers differ in their understanding of the concept mostly according to their subject traditions. Social science teachers emphasize social dimensions and science teachers' ecological dimensions, respectively. Teachers are aware of the relevance of the three dimensions to various degrees, but do not generally have a holistic understanding. The greatest uncertainty in teachers' understanding is related to the economic dimension. Science and social science teachers are critical of incorporating economic growth into the concept of sustainable development while language, vocational, and esthetical-practical teachers are not. No experience-bound differences of the teachers' understanding could be found, but recently qualified teachers consider their understanding of sustainable development to be poorer in comparison with more experienced teachers' self-evaluation. The study highlights the need for further training in sustainable development since more than 70% of the questioned teachers stated that they need such training.