In a context of increasing demand for quality and equity in education and a sharp focus on accountability, classroom teachers are also expected to support and improve learning outcomes for pupils in response to their individual needs. This paper explores three issues: how teachers understand assessment in relation to their students' learning, the curriculum and their pedagogical choices; how teachers' capacity to use assessment to improve students' learning can be developed through career-long professional learning (CLPL); and how teachers' learning can be implemented and sustained in schools, both locally and nationally. In considering these issues, recent thinking about learning and assessment and CLPL are considered alongside empirical evidence from the development and implementation of assessment processes and approaches to professional development in Scotland. The paper emphasises the importance of a dynamic framework of CLPL that recognises the individuality of teachers' learning needs and the consequent need for tailored professional learning opportunities with different combinations of support and challenge at school, local and national levels.