Abstract: This paper discusses the development and implementation of a particular tool─cases and case discussions─for the professional development of teachers of mathematics at the secondary level. Case development and the use of cases to foster in-depth discussions about teaching and content knowledge offer a promising approach to help develop effective mathematics teaching practice in middle and secondary classrooms. The appendix to this article includes a case that illustrates how this tool describes, as completely as possible, practice in a mathematics classroom. The case presents multiple perspectives, including the views of the teacher, the students and other adults in the school. The case offers a depiction of a common teaching dilemma and represents a powerful tool for professional development.

Key words: Mathematics teachers’ development; Mathematics teaching practices; Case discussions