Teachers’ participation in collaborative curriculum development is considered as having great potential for creating materials which are suitable for their specific context and for their professional development. However, the process in which teacher teams commonly negotiate the process of curriculum (re)design is not yet fully understood and conceptualized. Specifically, the manner in which they approach and execute the design task in a collaborative manner is seldom studied. In this study, 12 Teacher Design Teams (TDTs) in high schools in the Netherlands were followed during their effort to redesign their curriculum in the context of a school-wide reform. This yielded a rich description of their collaborative curriculum design, insights into the most conducive activities and insights into the function of textbooks in this process. Findings show that teams do not display a design process similar to existing and known procedural models. Design decisions and patterns are set early in the process and stay stable throughout. Collaboration in the design is aimed to the general decisions and not the construction of materials. However, the latter, when present, offers rich sources of learning and innovation. The measure to which the team has some common reform ambition at the start has a great impact on the process and result. The most productive activities during the process are those activities that assist teachers in creating a concrete image of their possible future practice. Finally, School leaders have a much more central role in supporting and guiding teacher teams in curriculum development than they often realize. They need to supply teams tailor made support based on the characteristics of the reform and the team.