The connection between teacher inquiry, professional development and school improvement was recognised 30 years ago by Lawrence Stenhouse. Stenhouse contributed many valuable insights into the role of practitioner enquiry in creating and utilising knowledge about teaching and learning, much of which is still to be applied systematically in teacher education and professional development. This paper draws on the Learning to Learn Phase 3 Evaluation, a three-year-action research project in which teachers in primary and secondary schools across the UK completed three cycles of practitioner inquiry to explore tools, pedagogies and other innovations which would promote dispositions of "learning to learn" (L2L). The paper focuses on identifying those aspects of being involved in L2L that support teachers' learning and the way that the teachers themselves understand the impact on their professional development. Data from over 60 semi-structured interviews undertaken over the three years of the project, the case study reports compiled by teachers at the end of each year of the project and collaborative workshops involving teachers and university researchers as co-inquirers are used to explore teachers' learning. Qualitative methods are used to develop a thematic analysis of the interviews, case studies and the teachers' understanding of the relationships between inquiry, research and continuing professional development (CPD) in order to identify categories and generate key concepts that can inform a theoretical understanding of the impact of professional inquiry on teachers' learning. The findings contribute to our understanding of the role of inquiry and research in schools in supporting professional learning by suggesting how tools and models of working are developed. (Contains 1 figure, 2 tables, and 1 note.)