For more than a decade, extensive government-funded school improvement programs have aimed at strengthening quality and equality in Swedish schools. Since 2015, a specific focus has been on the group of newly arrived migrant students in terms of in-service training for teachers. Research about such initiatives is, however, lacking, particularly regarding how these efforts help teachers to better attend to newly arrived students’ language and content learning in various school subjects. The overall aim of the present thesis is to contribute new and in-depth knowledge about subject teachers’ possibilities for developing a language and content integrated approach in their subject teaching for newly arrived migrant students at secondary school level (students aged 13–16). The study was conducted in a school in tandem with an in-service training on language and content integrated teaching and learning from a second language and multilingual perspective. Using a linguistic ethnographic approach, two secondary school teachers’ meta-reflections and teaching practice in Science and Mathematics are in focus. The rich material produced over two school years during and after the in-service training consists of audio-recorded interviews, the teachers’ collegial meetings during the in-service training, audio-and video-recordings from classroom observations, fieldnotes, and photos. Using an ecological and critical framework, the teachers’ perspectives are related to the material and structural context. Central to the analyses is the teachers’ situated didactic agency and how it is shaped by power relations, available resources, and circulating discourses. These analyses are supported by the interconnected model of teacher professional growth (Clarke & Hollingsworth, 2002), which is expanded via the use of didactic agency. Findings show that the school’s mainstream policy, combined with a lack of adequate resources, limits the teachers’ opportunities to fully make use of in-service affordances and, in the long term, their possibilities for practice development. The findings also make visible how intertwined and colliding discourses about teaching in general, and newly arrived students in particular, impact on the teachers’ actions and investments. This does not only affect what the teachers can do, but also what they can learn and develop further in their teaching practices. This thesis argues that what teachers do – and do not do – needs to be understood in relation to their didactic agency. Consequently, the present thesis problematizes school-improvement initiatives where professional development for teachers is constructed as a taken-for-granted solution to complex structural problems. The thesis further argues that in-service training for teachers cannot be regarded as a substitute for the lack of material resources and spaces for manoeuvre. These findings are relevant for future school-improvement initiatives as well as professional development initiatives, especially regarding marginalized student groups. In the thesis, research conducted in collaboration with professionals in schools is encouraged, where conditions and possibilities for teachers to design teaching practices that support linguistically minority students could be further explored. In such studies, teachers’ didactic agency would serve well as a central analytical concept.