The main questions of the research are: the proportion of teachers encountering multilingual children, what knowledge these teachers have about the language development of these children, where this knowledge comes from, and what practices they currently use in the teaching and learning of multilingual children. I designed this research as an experimental study to explore and understand Hungarian teachers’ knowledge about multilingual children and the pedagogical aspects of multilingualism. The research also investigated the willingness of teachers to increase their knowledge about multilingual children.