Assessment for learning is increasingly part of accepted orthodoxy, with massive government funding in England, is central to national assessment in Wales, and an export to the USA. Black et al.'s Assessment for learning: Putting it into practice (2003), the "bible"' of assessment for learning, is set reading for trainee teachers across the UK, and this text is increasingly a staple diet for all interested in assessment for learning. As such it has an important impact on all involved in the teaching and learning process. Despite this, there has been little discussion of either the paradigm or the definitions which inform it. This article examines the definitions of formative assessment and the theoretical premises of assessment for learning exemplified here and how they impact on the practices described. It finds a lack of alignment and coherence in the rationale of the theory, and contradictions which ensue in the practice. One solution is a paradigm shift basing definitions of formative and summative assessment on processes of assessment and not on functions. Functions remain as a basic epistemological premise of assessment. (Contains 1 table.)