Assessment as integral to good teaching and learning has come to be accepted as a core principle underlying curricula in many educational systems around the world. Indeed, the evidence that high quality assessment can make a big difference to pupil achievement, especially the low achievers, is mounting up (see, for example, Black & Wiliam, 1998) and provides a strong justification for those who promote assessment in schools. However, it seems that many teachers’ assessment skills (what we may call teachers’ assessment literacy) need to be improved. I will begin this paper by examining some of the international evidence for the last assertion and then proceed to outline an agenda for professional development in assessment that acknowledges the place of both classroom assessment and official assessment in supporting teaching and learning.