This article explores changes in teachers' beliefs and practice concerning assessment after participating in a project for improving assessment practices in Norwegian schools. The project was initiated by the Norwegian Directorate for Education and Training in 2008, and included a total of 77 schools, more than 600 teachers and a sample of their students. The main idea was to develop formative assessment practices in the classroom in the form of distinct criteria for clarification of how to reach curriculum goals. Furthermore, the project intended to increase students' motivation by using such formative assessment practices. The data presented in this article is from the research evaluation of the project. So far, the main challenge seems to be how to develop assessment criteria and how to use them in practice. (Contains 5 tables.)