An educational assessment embodies an argument from a handful of observations of what students say, do or make in a handful of particular circumstances, to what they know or can do in what kinds of situations more broadly. This article discusses ways in which research into the nature and development of expertise can help assessment designers create tasks that address key aspects of targeted learning. Four themes from expertise are presented: organisation of knowledge, social aspects of expertise, knowledge representations and interaction. Ways to develop assessment tasks around these themes are illustrated with examples from applied projects. (Contains 10 figures.)