This chapter is a description of collaborative action research on teacher competence in the context of inclusive education and universal design for learning (UDL). Our goal was to analyse what kinds of professional competencies teachers need when they are implementing UDL in heterogeneous classes. The action research was carried out as a case study together with two co-teachers and a class teacher, who implemented UDL in their heterogeneous classes. As a theoretical framework for teacher competence, we used the multidimensional adapted process (MAP) model of teaching, developed by Finnish researchers and teacher educators. We identified several teacher skills that are needed when the UDL approach is applied. According to our results, the most overarching necessary competence was the teachers’ cognitive skills. Applying UDL required the ability to flexibly transform one’s own teaching and learning situations. Furthermore, the teachers’ social skills appeared as an important attribute as their pupils were highly heterogeneous with diverse needs, and the teachers needed to collaborate with many other professionals and parents. Also, each teacher’s personal orientation, such as values, beliefs and ethics, played a crucial role in UDL while the teachers shared a common set of values, striving towards inclusive education.