Global mobility and economic and political crises in some parts of the world have fuelled migration and brought new constellations of ‘cultural diversity’ to European classrooms (OECD 2019). This produces new challenges for teaching, but also for assessment in which cultural biases may have far-reaching consequences for the students’ further careers in education, occupation and life. After considering the concept of and current research on ‘culturally responsive assessment’, we use qualitative interview data from 115 teachers and school leaders in 20 lower secondary schools in Austria, Ireland, Norway and Turkey to explore the thinking about diversity and assessment practices of teachers in the light of increasing cultural diversity. Findings suggest that ‘proficiency in the language of instruction’ is the main dimension by which diversity in classrooms is perceived. While there is much less reference to ‘cultural differences’ in our case studies, we found many teachers in case schools trying to adapt their assessment procedures and grading in order to help students from diverse backgrounds to show their competencies and to experience success. However, these responses were, in many cases, individualistic rather than organised by the school or regional education authorities and were also strongly influenced and at times, limited by government-mandated assessment regimes that exist in each country. The paper closes with a series of recommendations to support the further development of a practicable and just practice of culturally responsive assessment in schools.