At a time when much attention is being paid to teachers’ effectiveness, there is little regard for the effectiveness of their professional support. Although professional development facilitators are frequently involved in school improvement projects, little is known about the interventions they should carry out and the effectiveness of these interventions. In this study, five facilitators’ interventions are operationalised. Multilevel regression analyses show, that the intervention “guiding the process” explains a significant part of variance in teachers’ knowledge, attitude and concerns with respect to an innovation and the degree of implementation. The interventions “team training and coaching”, “creating conditions for innovation at school level” and “individual coaching” explain a significant part of variance in teachers’ knowledge with respect to an innovation. In general, it appears that professional development facilitators have considerable influence on teachers’ knowledge and concerns and reasonable influence on teachers’ attitude and the degree of implementation.