Teachers involved in action research in mathematics classrooms typically focus on their pedagogical practice, and rarely challenge the mathematical content of their teaching. The first part of this article supports this claim in terms of an analysis of the teachers-as-researchers movement. It then reports on a study where teachers were researching aspects of mathematics for the teaching of limits of functions, in which mathematical and pedagogical issues were intertwined. The aim of the study was to analyse how teachers’ mathematical knowledge evolves through their participation in a research project. The study showed that whilst their knowledge evolved for some aspects of limits during the research process this was not the case in two crucial aspects of this knowledge: the ε-δ definition and the graphical representation of limits. Furthermore, the study revealed the discomfort of an experienced teacher when facing challenges to his own mathematical knowledge and therefore the content of his teaching.