This dissertation is about developing our knowledge of teaching and student learning through examining teaching that has been designed and implemented with in a learning study. Applying an interdisciplinary approach, this research builds knowledge of how teaching can be designed and implemented for young students with different prior knowledge of mathematics. The focus is on inclusive education and, in particular, on inclusive learning environments. Teaching and learning about whole numbers for students in grades 2 and 3 were studied.The empirical material, mainly comprising videotaped lessons and the results of identical knowledge tests that the students completed before and after the lessons, was collected in a learning study and used in two steps. First, the teaching characteristics were analysed at a collective level, mainly based on test results used to measure learning outcomes, i.e.,students’ ability to expand their number range from natural to whole numbers. The extent to which the teaching characteristics were transferable to new teaching contexts was also investigated. Second, an interdisciplinary approach was followed, applying knowledge and theories from special needs education and mathematics education. Examining teaching characteristics in relation to outcomes, the research perspective expanded from the collective to the individual level, emphasizing participation and learning among students with different prior knowledge according to test results. Notably, at a collective level, teaching was characterized by being based on the identification of critical aspects. Analyses of teaching outcomes at the individual level revealed a mixed picture of student learning and participation in the learning study lessons. Concerning one studied lesson, an analysis of student test results revealed considerable diversity in student achievement, indicating that some students learned what was intended while others did not. However, test results from another lesson indicated that all students progressed in learning about whole numbers. During this lesson, the teacher-guided mathematical discourse incorporating several routines seemed to be central to facilitating student participation as well as change in the discourse on whole numbers.The main conclusion is that the interdisciplinary research approach helped foster deeper insight into teaching design and its outcomes regarding individual students’ learning and participation. In addition, the findings emphasize the importance of teachers taking responsibility for inviting the students into the mathematical discourse in relation to the access aspect of participation, and then, through challenging questions, facilitating progress to the collaborative aspect of participation.