Objective: In the past decade, several authors have advocated that formative assessment programmes have an impact on teachers' knowledge. Consequently, various requirements have been proposed in the literature for the design of these programmes. Only few studies, however, have focused on a direct comparison between programmes with respect to differences observed in their effect on teachers' knowledge. Therefore in this study we explored the impact of three formative assessment programmes on teachers' knowledge about supporting students' reflection. Methods: Our study was carried out in the domain of vocational nursing education. Teachers were assigned to an expertise-based assessment programme, a self-assessment combined with collegial feedback programme, or a negotiated assessment programme. We scored the verbal transcriptions of teachers' responses to video vignette interviews in order to measure their knowledge in a pre- and post-test. Multilevel regression analyses were performed to investigate differences in teachers' knowledge between the three programmes on the post-test; potential moderating effects of pre-test scores, contextual and individual factors were controlled for. Findings: The knowledge of teachers participating in the expertise-based assessment programme was significantly higher than that of teachers participating in the self-assessment combined with collegial feedback programme. Furthermore, the findings indicate that for professional learning, not only the approach to formative assessment is an important variable, but also the extent to which (a) teachers are intrinsically motivated and (b) they experience a high degree of collegiality at their school.