This paper reports on a study conducted as part of a larger project, named VIDEO-LM, which centres on video-based professional development for secondary mathematics teachers. The project aims to facilitate the reflective skills and the Mathematical Knowledge for Teaching (MKT) of secondary school teachers, in particular those teaching advanced mathematics courses. At the core of the project is a 6-component framework developed for analysing videotaped lessons in collabora-tive discussions with teachers. We describe the rationale and novelty of the project and the framework. Then, we focus on a study which examines the MKT growth of a group of teachers who participated in VIDEO-LM peer discussions, and present some preliminary findings.