This article presents an example of teacher professional development based on a perspective of situated learning and implemented on a large scale. We consider teacher professional development from three perspectives. First, teacher professional development is a key factor in improving classroom instruction. Second, teacher professional development is a vehicle for conveying knowledge from research into classrooms. Third, teacher professional development is an object of research itself. A German project to improve science and mathematics teaching (SINUS)--comprising 180 schools in a pilot-phase and more than 1,700 schools in a second phase of scaling-up--serves as an example of this framework for teacher professional development. Using these three views we describe the foundations of the programme and provide a brief account of the programme's background and its conception. We show how the central elements of the programme (11 modules) are based on an in-depth analysis of science and mathematics education, as well as how those modules structure the professional development of the teachers. Finally, we provide an overview of the evaluation of the programme. A large-scale comparison between SINUS schools and a representative sample of German schools tested in PISA 2003 showed positive effects of the programme with regard to students' interest and motivation as well as competencies in science and mathematics. In the light of these findings, we argue that teachers' learning related to daily pedagogical challenges in the classroom should be central to professional development initiatives. (Contains 3 notes and 2 tables.)