Purpose: This paper aims to provide a holistic approach to assessing student teachers’ competence development in education for sustainable development (ESD). This is to provide evidence on which teaching and learning formats help to foster which aspects of ESD-specific professional action competence in teachers. The studied competencies consist of content knowledge (CK), pedagogical content knowledge (PCK) and the willingness to actively support and implement ESD. Design/methodology/approach: A multiple case study design was used on two sequential modules of a university’s teacher education program. A mixed-methods approach was applied that combined surveys, videotaped and PhotoVoice-supported focus groups, as well as pre- and post-assessment tools. Qualitative data analysis was based on the coding paradigm of the qualitative content analysis, whereas quantitative data were interpreted by means of descriptive statistics and paired sample t-tests. Findings: The results from this study clearly indicate that the two courses contributed to a shift in students’ non-cognitive dispositions. The study also provides evidence on the students’ competence development and demonstrates how two different learning settings support different dimensions of teachers’ professional action competence in terms of ESD. Originality/value: The triangulation of data enabled not only a mere competence assessment but also deeper insights into learning processes, as well as into the drivers of and barriers to competence development. Furthermore, the study introduces an innovative approach to assessing the development of PCK.