The focus of this chapter is an analysis of multilingual interaction taking place at three types of secondary schools in the province of Friesland, the Netherlands. The aim is to explore whether multilingualism is used to learn, or whether teachers learn to act multilingually. It focusses on a multilingual education project in which activities in several languages were developed. The project had a bottom-up approach in which research questions from the schools regarding multilingualism formed the basis for the development of multilingual activities. These were implemented, evaluated, and optimised during three cycles of design-based research. The project and the developed activities were analysed through interviews with teachers which sought to gauge their beliefs on the use of multiple languages in teaching. Also used were classroom video-recordings which enabled a greater focus on the forms of interaction used in the classroom. For this purpose, an adapted version of a framework by Gajo and Berthoud (2018) was used to analyse two main parameters: language regime-meaning language alternation at micro-, meso-, and macro-levels; and participation regime-meaning the ways in which pupils are addressed and involved in educational contexts. The results show examples of the role of multilingualism in learning, as well as ways in which teachers learn to act multilingually by drawing on pupils’ home languages.