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".