Supporting Teacher Design Teams (TDTs) during local curriculum development efforts is essential. To be able to provide high-quality support, insights are needed about how TDTs carry out design activities and how support is valued by the members of TDTs and how it affects their design expertise. In this study, the design and support processes of two TDTs assisted by an external facilitator were investigated using a case study approach. The results revealed that support offered to TDTs depended on the contextual boundaries and the focus of the design process. The focus, coherence and form of support affected the opportunities for developing teachers' design expertise. In both cases, teachers' curriculum design expertise had grown, whereas their pedagogical content knowledge and subject matter knowledge were hardly developed. Findings show that the most conducive support activities and materials were those that could directly be applied in the design process.