In the present thesis, four studies are included within the scientific discipline of pedagogy and collaborative professional development research field—the thesis is based on a practice-based research approach. This thesis addresses identified problems in previous research and large-scale studies such as the OECD study Teaching and Learning International Survey (TALIS) and school leaders’ and teachers’ attitudes. Furthermore, the need to design teaching that is content-inclusive and accessible to all students. The study’s overall question is: What factors of importance for promoting content-inclusive teaching emerge in the studies? Since the thesis aims to contribute with knowledge of how collaborative professional efforts can support teachers’ understanding of developing inclusive teaching in the classroom regarding the accessibility of teaching content for all students, the theoretical perspective is motivated by a cultural-historical perspective as an overall theoretical framework. Within the cultural-historical perspective, situated learning and communities of practice (CoP) are in the foreground. The framework of variation theory has been chosen to analyse which teaching content the students are offered to distinguish. The articles used both specific subject content to show how conditions for inclusive teaching can contribute to increased availability of the content knowledge and more comprehensive studies of how teachers’ collaborative competence can contribute to increased accessibility. Finally, teachers’ perspectives and experiences are in the foreground. The design of the four studies is based on a mixed-methods approach. That means the design of the thesis used qualitative and quantitative methods, both individually and in combination, in the different studies. Since two-year time perspective, it is defined as a longitudinal process. The synthesis describes the factors necessary for promoting content inclusion that has been highlighted in the dissertation studies. Three factors have been identified in the dissertation’s results of significance for how collaborative professional efforts can support teachers’ understanding of developing inclusive teaching in the classroom regarding the teaching content’s accessibility for all students: 1. Development of CoP with a focus on content-inclusive teaching 2. Collaborative professional development that challenges teachers’ views on inclusive teaching 3. Analysis of intentional and enacted teaching to identify what is inclusive Within the framework of the studies the dissertation consists of, the results show that the cyclical classroom models, Lesson and Learning study, have contributed to the teachers gaining an in-depth understanding of which aspects have been particularly successful in developing their teaching design. The studies show factors that are important for promoting content-inclusive teachings, such as when the entire teaching team takes collective responsibility for content-focused teaching and is given time for collaborative professional development. Another factor is that teachers are given the opportunity to develop their ability to analyse how teaching can be planned and implemented to identify what contributes to inclusive teaching, where the content and learning situation is put in the foreground. This change has meant a higher focus on the availability of teaching content to all students, compared to a previous focus on individual students’ need for special support. The present dissertation results can be seen as a contribution to practicebased professional development research where researchers’ and teachers’ joint competence is the basis for systematically and structurally approaching the object of learning, thereby reducing the gap between theory and practice.