<< Background >>Many European schools still fail to provide equitable access to high quality mathematics education for all students, regardless of the students’ backgrounds. Driven by the increasing language diversity and persisting opportunity gaps in Europe, the project aims to develop professional development (PD) opportunities for teachers to develop their expertise in being responsive to linguistic diversity in mathematics classrooms. Being responsive to linguistic diversity means to treat language aspects as explicit learning goals and to build upon multiple languages as resources for mathematics learning. As language is an important thinking tool for constructing new mathematical knowledge, language responsiveness is essential if students are to gain equitable access to mathematics, regardless of their linguistic backgrounds.A fruitful means of being responsive to linguistic diversity in mathematics is the activation of multiple languages in mathematics classroom for the purpose of exploiting the connection of languages as learning opportunities. For instance, in Turkish, the fraction 3/5 is expressed as “5, therein 3”. Comparing this expression to the English “three fifths” can initiate deep learning processes on differences in conceptualizing fractions, not only for Turkish-speaking students, but for students of all linguistic backgrounds. These multiple languages can be either students’ home languages and the language of instruction (e.g., German), or English as Medium of Instruction in bilingual education (e.g., in the Netherlands), or combinations of both. Engaging students in these kinds of deep learning processes requires teachers’ expertise and specifically developed educational resources.<< Objectives >>The objective of the project is to design professional development (PD) opportunities that enable mathematics teachers to be responsive to linguistic diversity in mathematics classrooms.To achieve the project objective, therefore, the project has the following goals:1: Design and validate digital Open Educational Resources (OER) that are responsive to linguistic diversity, as material support for PD opportunities.2: Design and validate PD opportunities to facilitate mathematics teachers’ expertise in harnessing the epistemic potential of multiple languages, through their implementation and reflection of said OER.3: Examine the effects of the material support and the PD opportunities on mathematics teachers’ expertise for being responsive to linguistic diversity, also comparatively across the diverse language constellations in the national contexts of the project partners.<< Implementation >>The project is organized in two phases.Phase 1 of the project is concerned with developing the material base for PD opportunities, firstly in the form of digital OER that are responsive to linguistic diversity, for the topic of algebra (generalizing patterns, equations), and secondly, in the form of designing –together with 3x5 teachers– reflective activities with the OER for facilitating teachers to perceive, utilise and reflect on exploiting the epistemic potential of multiple languages.In Phase 2, PD opportunities are designed to facilitate teachers to develop their expertise for being responsive to linguistic diversity in the mathematics classroom. In the three countries, 3x15 mathematics teachers will be engaged in international Professional Learning Communities (PLCs), organized around the teachers’ own experimentation with the digital OER in typical cycles of collective inquiry:(a) appropriating the principles and aims of the teaching approach in the OER,(b) analyzing and adapting the OER for own experimentations,(c) experimenting in their own classrooms,(d) collectively reflecting on the classroom experiences and students’ products.In these cycles of inquiry, teachers experience the benefits and pitfalls of exploiting the epistemic power of multiple languages in the classroom. Through their reflective activities, they learn to identify moments that are particularly suited to exploit the connectedness of multiple languages for supporting their students’ conceptual understanding.The project will utilise qualitative methodologies for investigating teachers’ reflective processes about harnessing the epistemic potential of linguistic diversity in their mathematics classrooms, with a particular focus on teachers’ learning pathways. In particular, data on the teachers’ adaptation and implementation of the material, their reflection on these adaptations, as well as on students’ learning and language use will be collected. The data will be analyzsed with specific methodologies to account for the multilingual context, as developed in earlier projects of the partners.<< Results >>Findings of this project will improve our understanding of how to facilitate mathematics teachers to be language-responsive towards the inclusion of multiple languages in mathematics classrooms, and to harness multiple languages for supporting all students’ mathematics learning irrespective of their linguistic backgrounds.The PD module, as the central output of the project, will have an impact on several levels. Firstly, at the level of the partner institutions, all partners can integrate the PD module into regular teacher education programs for preservice teachers, as well as into their regular PD courses for in-service teachers, potentially reaching hundreds of teachers and, in turn, improving the mathematics learning of many more students. Secondly, on the school level, schools will have the opportunity to sustainably work with the developed teaching materials in their classrooms and expand the group of involved teachers within the schools though collegiate professional development. Thirdly on the policy level, regular policy papers inform educational policy makers about these positive examples for how to include multiple languages in education. The research evidence from this project about how linguistic diversity can deepen subject matter learning is essential, as it can motivate other educational institutions to utilise language diversity to improve student achievement in different subjects.