Given the challenges of scaling up content-related professional development (PD), the PD facilitators have gained increasing attention in PD research. In this mainly programmatic and structural article, we try to systematize existing research strategies which take into account the multifaceted and multi-level structure of PD research. We discuss the lifting strategy which draws upon structural analogies between the classroom and the PD level, the nesting strategy for research-based design, and the unpacking strategy for design-based PD research and locate these strategies within a Three-Tetrahedron Model for PD research. Thereby, we present a framework to systematize and explain existing research approaches and to identify necessary but missing research contributions. The article provides a language for initiating a discourse on research agendas which can support PD facilitators: The Three-Tetrahedron Model for PD research and design captures the complexity of learning and teaching at the classroom, teacher, and facilitator level that is needed to inform design and research into PD as well as to uncover gaps in the literature.