This paper describes a research study into the development of professional knowledge through action research by teachers. The research addressed three questions: (1) What is the nature of the knowledge that teachers develop through action research? (2) To what extent and in what way do teachers develop this knowledge? and (3) To what extent and in what way do facilitators have an influence on this? The study is founded on the Anglo-Saxon tradition of action research and the German Allgemeine Didaktik. It was designed as a descriptive and explorative case study. The results show that teachers first concentrate on the 'technological' area of knowledge, followed by the 'empirical' and 'ideological' areas of knowledge. They mainly reflect on what they do (phenomena) and not on what they achieve (effects). Left to themselves they are also more inclined to focus on desirable action rather than real action. It emerged however that teachers develop knowledge in all three areas when facilitators pay attention to these areas.