This article describes some outcomes of a nine-month design-based research study into the professional development of 24 numeracy teachers with post-16 learners. Teachers analysed research-based principles for teaching, and engaged in a design-research process by testing and refining teaching activities to embody these principles. Data from questionnaires, interviews and classroom observations suggest that many of the teachers' practices and beliefs were profoundly affected. Practices became less transmission-oriented and teachers began to create new, collaborative learning environments where students were challenged to confront difficulties and take on more active classroom roles. Changes in beliefs regarding mathematics, learning and teaching are also described and discussed. (Contains 2 tables, 2 figures and 2 notes.)