In a context of increasing demand for quality and equity in education and a sharp focus on accountability, classroom teachers are expected to support and improve learning outcomes for students in response to their individual needs (OECD, 2013). To sustain such improvements, they need strong theoretical frameworks for assessment and evaluation that will help them to make principled changes to their practice (Mansell and James, 2009). However, they often hold deeply rooted views on learning and assessment that can inhibit change, finding it challenging to understand the complicated and dynamic interactions amongst curriculum, assessment and pedagogy and the learning environment to support students’ learning. Many teachers continue to think of assessment as separate from, rather than integral to, pedagogy and focus on coverage of curriculum content rather than on understanding and assessment of progression in knowledge, concepts and skills and their application. Support for teachers to uncover and confront their prior beliefs about assessment and evaluation and reflect on the impact they have on pedagogy is seldom an integral element of professional development (Livingston, 2015). The changes needed to develop teachers’ capacity in assessment involve understanding more about them as individual learners and the conditions that best promote their learning, as well as their students. The paper will therefore explore three related 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’ CLPL can be implemented and sustained in schools, both locally and nationally. In considering these issues, recent thinking about learning and assessment and career-long professional learning will be considered alongside empirical evidence from the development and implementation of assessment processes and approaches to professional development in Scotland in the context of curriculum, assessment and pedagogical change (Hutchinson and Young, 2011; Livingston, 2012). The discussions about teachers’ learning will draw on socio-cultural theories of learning (Vygotsky, 1978); adult learning theories (Mezirow, 1997); professional learning communities (Wenger, 2010); and peer-mentoring (Livingston and Shiach, 2013). The results of the analysis will be presented with a particular focus on identifying the conditions in schools necessary to build teachers’ capacity in assessment through career-long professional learning through system, culture and practice change.