This paper explores the value of action research in the understanding of the professional development of 4 preservice secondary school mathematics (PSSM) teachers and their instructors in a teacher education program. It draws upon data collected from multiple cycles in a research project (2007-2010). The goal of the long term project was to explore how PSSM teachers combined knowledge from their content area and pedagogy courses with the action research course. This paper represents how activity theory facilitated the instructors' interpretations and understandings of the benefits and challenges of using action research as a professional development strategy with PSSM teachers. (Contains 3 figures and 1 table.)