Although action research is widely acknowledged to have benefits in terms of improving practice and professional development, its ability to generate new knowledge, and hence its status as research, is debatable. Indeed, there are questions as to whether it can be called "proper" research. This article draws on the Southampton Music Action Research Project, 2007-08, to examine how seven secondary school music teachers undertook practitioner research projects in England, and what knowledge their projects generated. It finds that this knowledge included experiential, presentational, propositional and practical knowing. Although such knowledge is positioned as "Little K" knowledge, the reception accorded to it by other teachers suggests that knowledge, generated by teachers' action research, might sometimes have potential to be accepted as "Big K" knowledge. (Contains 2 tables and 1 note.)