The National curriculum assessment (NCA) in Wales has evolved from common foundations into a system that is now distinct from that in England. The influence of the political and social milieu of Wales can be seen both in the distinctive features that have been in place from the outset and in the more radical changes introduced since 2002. In this commentary, the author discusses distinctive features of NCA and characterises the experience of national curriculum assessment in Wales in the 1990s in terms of "adaptations" to a testing regime and to the uses of performance data that were designed and managed in a policy environment dominated by the prevailing social and political milieu in England. The "divergence" in assessment policy and practice since 1999 is the product of a distinctive policy environment released from the constraints of control from London. The author stresses that the current changes to assessment policy and practice in Wales rely on the professional expertise of teachers, both through assessment for learning practices in their classrooms and in teachers' ability to make valid judgments of attainment that are reliable enough for the purposes for which summative data is required. (Contains 3 notes.)