Dialogic Teaching (DT) is effective in fostering student learning; yet, it is hard to implement. Little research focused on secondary teachers' learning of DT and on the link between teachers' understanding and practices, although these two are usually strongly intertwined. Using a wide range of evidence, this case study systematically investigated and compared two secondary teachers' understanding and practice of DT during their participation in a continuing professional development programme (CPDP). The CPDP appeared effective to some extent. The History teacher's understanding of DT, i.e. being a co-learner, appeared highly effective in implementing DT, whereas the Mathematics teacher's understanding of DT, i.e. creating a democratic learning environment, seemed only effective to some extent. Focusing on both teachers' understanding and practice when developing DT seemed fruitful in explaining differences in practice. Future research could further explore to what extent understanding DT as being a co-learner facilitates professional development.