In spite of the widely accepted need for language-responsive subject-matter teaching, few teachers are prepared for this challenge due to the lack of empirically founded subject-specific professional development (PD) programs for language-responsive classrooms. The design research study presented in this article pursues the dual aim of (a) promoting teachers’ expertise in language-responsive mathematics teaching using PD courses and (b) investigating teachers’ developing expertise in qualitative case studies. Both aims are pursued based on a conceptual framework for teacher expertise in language-responsive mathematics teaching, starting from typical situational demands that teachers face in language-responsive mathematics teaching and the orientations, categories, and pedagogical tools they need to cope with these situational demands, especially the demand to identify mathematically relevant language demands. For language-responsive teaching, the interplay of categories for mathematical goals and language goals turns out to be of high relevance.

Learning situations that concentrate on conceptual understanding are particularly challenging for learners with limited proficiency in the language of instruction. This article presents an intervention on fractions for Grade 7 in which linguistic challenges and conceptual mathematical challenges were treated in an integrated way. The quantitative evaluation in a pre-test post-test control-group design shows high effect sizes for the growth of conceptual understanding of fractions. The qualitative in-depth analysis of the initiated learning processes contributes to understanding the complex interplay between the construction of meaning and activating linguistic means in school and technical registers.

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.