Teaching sustainability topics puts teachers in a tense relationship: on one hand, they should convey the importance and urgency of the topic, but at the same time, this should be done in such a way that students can form their own judgements. In addition, teachers may themselves become privately involved and interested in the topic, which requires a professional understanding of their role. This concern was pursued in the present study. Teachers who teach sustainability topics were interviewed in semi-structured interviews. This article presents the results by means of case studies that show three contrasting cases. There are two ways of avoiding the tensions and one that takes the tension as an inherent part of the topic.