Research has shown that how principles and tools related to Assessment for Learning (AfL) are taken up by teachers can significantly impact how assessment initiatives are realised. However, questions remain about how teachers develop new resources for use in local contexts, and what these processes entail. This article examines such processes in the context of teacher teams, using the notion of "knowledge work" to denote the creative and constructive work that teachers carry out when they work together upon AfL-related principles and tools. The analyses contribute to existing research by illuminating the micro-dynamics of teachers' work with assessment resources, and examining different forms of epistemic actions that constitute this process. In conclusion, the paper discusses how teacher collaboration may inform AfL initiatives in significant ways, and argues that teachers' constructive and collaborative knowledge work needs to be more fully recognised in the context of AfL initiatives.