Today’s educational environment is increasing multilingual where teachers and students alike are communicating and learning in multiple languages. The experience is often challenging for these students who, at the same time, are learning the language of instruction and are expected to develop subject area knowledge. Teacher beliefs about and assessment of these students’ learning is thus of particular importance when considering this phenomenon and, as of yet, is a largely unexplored research area. The aim of this thesis is to contribute to this research area and draw attention to the consequences varying language policies and pedagogies may have on fairness and validity in assessment outcomes. This thesis includes two studies on teachers’ assessment beliefs and reported assessment practices in Swedish schools: - In Content and Language Integrated Learning (CLIL) context, Grades 10-12. - In contexts with newly-arrived migrant students (NAS), Grades 7-12. Helena Reierstam Taught English and French as a foreign language in Swedish upper secondary schools and ESP in Higher Vocational Education. More recently she held a position as a Dir. of Ed. at the Swedish National Agency for Education where she worked with national tests. Doktorsavhandlingar från Institutionen för pedagogik och didaktik 65 This is a Doctoral Thesis in Education at Stockholm University, Sweden 2020