Providing cognitively activating learning opportunities in the mathematics classroom is essential for instructional quality and the development of competencies of students. Correspondingly, the teachers’ awareness of big ideas in mathematics is a goal shared by the aims of national standards in many European countries. By ‘big ideas’ we are referring to such notions as: infinity, symmetry, thinking with images, counting, mapping, statistical inference, rather than merely procedures for calculating. A crucial base for knowledge about such overarching concepts is mathematical content knowledge and corresponding so-called pedagogical content knowledge. However, in many countries sufficiently deep content knowledge in mathematics is not made available in pre-service teacher training, and in-service learning of teachers is often inhibited by the everyday constraints of the workplace on the one hand and the inertia of established classroom routines on the other.Consequently, the project aims at making innovative solutions accessible for in-service mathematics teachers, taking into account the inhibiting factors. The product of the project will be an online forum containing content resources for teachers linked to big ideas, pedagogical content knowledge for creating rich learning environments, and hypermedia video cases. These videotaped instructional situations enriched with accompanying material will be linked to big ideas and support reflections of teachers about classroom practice. The sustainable dissemination of the outcomes will not only include the established close partnership with schools of the project teams, but also focus on a wider-spread community of teacher teams in the partner countries and beyond. A further key element of the project will be the evaluation research, which will contribute to knowledge about characteristics of effective teacher training on a European level. The evaluation will follow the competency approach of the Lisbon initiative.