This article describes the design and results of a descriptive and explorative case study into the development of professional knowledge by teachers through action research and the facilitation of this by teacher educators. The theoretical framework of the study links the Anglo-Saxon Action Research tradition and the German 'Allgemeine Didaktik'. It distinguishes three domains of knowledge: ideological, the domain of educational norms, values and objectives; empirical, the domain of connections between phenomena in educational reality; and technological, the domain of educational methods, techniques and strategies. Seven groups of teachers at six schools participated in the study. Teacher educators facilitated them over a period of two years. The findings indicate that left to themselves, teachers tended to develop knowledge mainly in the technological domain and rarely in the empirical and ideological domains of knowledge. The findings also show, however, that the more attention facilitators paid to all three domains of knowledge, the more teachers started to develop knowledge in those domains.