This article focuses on teachers' learning in a research and development (R&D) work project. The teachers are working in a lower secondary school, and the cooperation between them and the researcher lasted for two years. The aim of this article is to show what teachers can learn and how they can learn during such a project. In the qualitative study conducted within this R&D project, observations, reflection dialogues and log-book entries were key data collection strategies. This material shows that teachers want to reflect together to learn, but both the form and content of these collective reflections have a major impact on their learning. Before the teachers are ready to challenge each other, they have to have confidence in each other. Once they have this confidence, the form of the reflection dialogues can change. During the R&D work the teachers learn how reflections become part of the planning process for their next teaching sessions, and after a while theories become a tool that can help them with their reflections. (Contains 2 figures.)