From an inclusive perspective in mathematics education, the aim of this Ph.D. thesis is to gain profound knowledge of didactical and relational aspects of mathematics teaching concerning students’ participation in mathematics education. The overarching research questions delve into methodological and theoretical aspects that enable the study of enacted mathematical and relational knowing of teachers, as well as how mathematical and relational knowing supports inclusion in mathematics for students. Three studies were conducted. The first study investigated the possibilities of various data collection methods to document mathematics teachers’ reflections on mathematical and relational knowing in mathematics education. The second study aimed to gain theoretical and empirical insights into teachers’ mathematical and relational knowing, as well as into students’ participation in mathematics. The third study examined earlier research regarding how co-teaching organized mathematics education can contribute to inclusion. These three studies generated four publications that answered the overarching research questions. The methodological and theoretical findings emphasize the complexity involved in exploring inclusive mathematics education. Understanding mathematical and relational knowing of teachers requires abroad conceptual framework that considers how these forms of knowing come into play when teaching. The coordinated conceptual frameworks used have core elements connected to the situated nature of inclusive mathematics education. These frameworks are based on contextual factors that focus on how and when teaching materials, mathematical examples and connections are used. They also consider the particular teacher-student interactions that occur in the moments of teaching. Hence, from a special didactic perspective, it is essential to emphasize not only the ‘what, ‘how,’ and ‘why’, but also the ‘when’ question, considering both mathematical and relational knowing in the context of inclusion in mathematics. Furthermore, findings show that if students are to be didactically included, meaning they participate in mathematics within the community of classroom mathematics, it appears important for teachers to enact mathematical and relational knowing simultaneously, whether distributed between one or two teachers in a co-teaching setting. Moreover, co-teaching does not automatically lead to didactical inclusion. However, if all students’ learning is the point of departure and the teachers, together or individually, enact mathematical and relational knowing, this can contribute to didactical inclusion.