The increase in international migration and the global number of students learning mathematics through a language other than their home language makes migrants’ educational pathways a particularly urgent issue. The current paper focuses on the German context as Germany is currently one of the major immigration countries in the Western context. Because German is widely perceived as the language of opportunity for migrants in the German school system, it is overwhelmingly the selected language of learning and teaching in the broader school system and within migrant classes. The article is based on qualitative interviews with mathematics teachers teaching migrant classes; and the data analysis followed the Grounded theory tradition. The central phenomenon that seemed to preoccupy all teachers was how to adapt mathematics teaching to the language- and mathematics-related diversity of their students while simultaneously helping them to transfer smoothly to the mainstream classes or pass the final examinations successfully. Due to the lack of institutional support for teaching in migrant classes and problems in dealing with multilingualism and mathematics-related diversity in mathematics lessons, they were generally frustrated with the situation despite being strongly motivated at first. Using three examples from the data, we illustrate teachers' strategies in relation to multilingualism and mathematics-related diversity. Overall, the study points to the necessity of supporting teachers in their mathematical practices in migrant classes to contribute to more inclusive mathematics education.