This paper describes some outcomes of a nine-month design-based research study into the professional development of 24 numeracy teachers with post-16 learners. The teachers were encouraged to integrate eight research-based teaching principles into their classroom practices as they implemented a set of discussion-based mathematics learning resources. A mixed methods approach was used to evaluate the outcomes, including interviews and classroom observation. The paper examines teachers' perceptions of the project, the reasons they found some pedagogical principles more difficult to incorporate than others, and the factors that enabled and impeded their use of the learning resources. In particular, it is noted that the principles that teachers considered to be most important were not the ones that they were observed using most effectively. The paper concludes by considering the implications of the research for initial and continuing teacher education. (Contains 2 tables, 2 figures, and 7 notes.)