Informed by an amalgam of notions drawn from constructivist, socio-cultural, metacognitive and self-regulation theory, the discourse created to describe assessment to enhance learning has gone through a number of iterations or discursive shifts. As a result, the current discourse is both ambitious and complex with the roles and responsibilities assigned to teachers and learners in learning and assessment radically transformed. Sponsored by the New Zealand Ministry of Education, the Teaching and Learning Research Initiative (TLRI) seeks to develop teachers' research capability and to build knowledge about teaching and learning with the intention of improving outcomes for learners. Using a range of qualitative data generation strategies this TLRI research project investigated secondary school teachers' and students' conceptions of assessment and feedback. This paper reports on the professional learning that accrued for these teachers using the data gained from the four mathematics teachers in the project. It also details changes in these teachers' thinking about the roles and responsibilities of teachers and learners in the assessment process and documents reported changes to their professional practice resulting from these changed views. Whilst it is argued that involvement in the project became a valuable form of professional learning whereby these teachers, to varying degrees, achieved accessibility to, and greater understanding of, the current discourse of formative assessment, the mediating influence of teachers' efficacy beliefs is also acknowledged. (Contains 2 notes.)