Purpose: This study aims to investigate the role of professors as gatekeepers for sustainable development competencies (SDC) in disciplinary study programs. It aims to understand which factors are crucial for professors to integrate SDC into their teaching, their basic understanding of SDC, the actual degree of SDC integration and how suitable they perceive student-centered teaching strategies for teaching SDC. Design/methodology/approach: The results are based on the qualitative content analysis of interviews with 16 professors after they participated in a didactic training program focused on combining student-centered teaching strategies with SDC. Findings: Psychological factors, like the attribution of responsibility, value orientations and cost-benefit considerations, are not the only reasons for integrating SDC into a course, as disciplinary requirements play their role. When teaching SDC, professors focus on systems-thinking, strategic and normative competencies. They consider student-centered teaching strategies especially suitable to teach systems-thinking and interpersonal competencies. Social implications: Promoting changes toward teaching SDC may best be done by supporting professors' intrinsic motivations--by fulfilling the need for growth in teaching skills, activating values and creating an environment in which everybody feels responsible for teaching SDC. Originality/value: Teaching SDC with student-centered teaching strategies is relevant even in study programs that show little relation to sustainability issues. Professors are experts in their field, but not necessarily in the field of sustainability. Understanding how such professors might include sustainable development competency development in their syllabi can widen the influence of SDC on higher education.