All teacher education programmes require to handle complex layers of learning because they are teaching teachers to teach. Arguably a programme involving Content and Language Integrated Learning (CLIL), preparing experienced school teachers to teach curriculum subjects in a target foreign language (usually English), has an additional layer of complexity. This chapter describes an action research approach to the investigation of a CLIL teacher education programme and indicates that a pedagogy aligned to action research seems effective in such contexts. There is an indication that action research based pedagogy applied to CLIL teacher education can lay the foundation for inquiry-based professional learning where teachers develop as practitioner-researchers who are able to improve their own practice and also apply research methods to systematically validate their work, thus contributing to the body of CLIL research.