Collaboration is a crucial element of effective professional development for teachers. In Teacher Design Teams (TDTs), teachers collaborate on (re)designing educational materials. To optimize their effectiveness, a strong theoretical and practical basis is required. In this study, therefore, we first developed a conceptual framework based on literature. Subsequently, we used a mixed methods approach to explore teachers' perceptions regarding the factors in this conceptual framework. The results reveal detailed insights in the functioning of TDTs, for example, regarding the role of the team coach. The resulting framework can be used to describe the functioning of TDTs and improve future TDTs.