This interview-based study investigated the impact of two academic development courses about assessment. Participants learnt concepts and terminology that enabled them to better understand and communicate about assessment. The courses also stimulated critical thinking about assessment practices and conceptual change, with 'assessment for learning' emerging as significant with the qualities of a threshold concept. Certain learning activities stood out as transformational as they supported the examination of assessment from different perspectives and the integration of implicit with explicit knowledge, but only if participants had sufficient workplace experience that they could link to the formal knowledge taught in the courses.