Education has responded to global Sustainable Development (SD) goals by drawing on a range of issuerelated, context-based, and cross-curricular approaches to teaching SD. This paper aims to examine how teachers describe the content in SD and how they justify this content concerning their teaching practice. The Pedagogical Content Knowledge (PCK) framework, with its reflective tool Content Representation (CoRe), was used in combination with semi-structured interviews to capture 18 Swedish upper-secondary science teachers' reflections. The results indicate four themes of content that the teachers emphasise: energy and global warming, ecosystem services and biodiversity, presence of harmful substances and materials in nature, and imbalance of natural substances in nature. The teachers justified these content themes in terms of the importance of students' gaining different perspectives (local, global, ecological, social, and economic), belief in the future, action competence, and general scientific education. The paper contributes to sustainability education research, as it focuses on teaching sustainability in the light of the PCK framework. Also, it can support and inspire teachers when they decide what content to include when teaching SD. © 2024 University of Oslo, Norwegian Centre for Science Education. All rights reserved.