This action-research consists in designing, implementing and evaluating the continuous teacher education programme ‘Ecole multilingue’. The programme is aimed at preschool and primary school teachers and concerns the pedagogy of multilingualism. During the programme, video excerpts showing language learning practices at school were studied, multilingual projects were designed and implemented in the classes of the participants and discussions and exchanges took place within ten monthly meetings. The research questions are the following: What is the professional development that is realised throughout the programme? What are the characteristics of the programme that have afforded this development? The research positions itself within the framework of socio-constructivist and socio-cultural theories. The chosen methodology draws upon thematic analysis, grounded theory, analysis of interactions and analysis of practice. It implies the collection of different types of data: questionnaire, videotapes and transcriptions of the training meetings, logbook of the trainer-researcher, videotapes and descriptions of innovative practices. During the research process, description and analysis grids are used and designed, and research results are cut across. A reflexion about the double role of a trainer and a researcher is developed. The analysis reveals the representations about language learning, multilingual practices and institutional context, as well as the ways of analysing pupils’ learning processes, co-constructed by the participants. It characterises the innovative multilingual practices that the participants have developed in their classes, and it shows the process of accompanying-challenging by the trainer. It furthermore brings out the ways in which the different elements of the programme have contributed to the teachers’ development. It finally may inform a prospective, improved training programme.