In this workshop the focus is on epistemological and methodological issues associated with research on teacher knowledge. The authors will first present some of the findings of their own research. In various projects, the authors have studied the nature and the development of secondary teachers' knowledge in The Netherlands. These projects, for instance, involved pre-service teachers of chemistry in the course of a post-graduate teacher education program and experienced language teachers who teach reading comprehension. In the presentation of these projects the emphasis will be on what it is that was investigated, and how this was investigated. Most of the time of the workshop will be used for group discussions of these issues, which are often problematic in research on teacher knowledge. The participants in the workshop, including the presenters, may benefit from sharing their ideas about and experiences with these issues.