We know assessment plays a major formative role in driving student learning appropriately, but what implications does this have for online courses? Is it more important than in a face to face context, or less so? Should we reconceptualize the ways in which we assess students, or are existing methods, tried and tested in conventional teaching and learning situations, appropriate? This paper discusses the practical implications of implementing these online pedagogies and illustrates the powerful formative effects, both intended and unintentional, of assessment on student learning and behaviour. It draws on examples from recent research on two e-learning courses at UK Open University. (Contains 3 figures.)