Today's society is multicultural which means that also our classrooms are multicultural. The teaching must then be designed so that all pupils can develop as far as possible, which is expressed in the curriculum. The purpose of this study is therefore to investigate how Swedish teachers work with language development in the mathematics subject, to support these multilingual students’ knowledge development to the maximum.This study has a qualitative approach as it is based on semi-structured interviews. A total of four teachers participated and were thus able to give their picture of how they work with language development in mathematics. The results of the study indicate a large divergence between the teachers' ways of working, relating to multilingual classrooms, and language development work. In addition, some aspects are more crucial to the development of languages and knowledge for multilingual students. This includes collaborative work where language has a central role in which students get to talk, reason, and communicate mathematics with others. Other key strategies that emerge are reality anchoring, customized tasks, and multilingualism as a resource. The work is analyzed in relation to theories of socio-cultural learning and pragmatism.