This thesis consists of two studies, described in two papers, and a re-analysis of data from these studies. The aim is to broaden the knowledge about what contributions a learning theory, variation theory (Marton, 2015), can give to teachers’ teaching practice. The studies have an action research approach (Elliot, 1991). Data was generated in the two studies with two different “teacher groups” having different learning goals for their students. Each group consisted of a researcher and five mathematic teachers. Variation theory was used as a theoretical tool to plan, teach and assess teaching in the context of a Professional Learning Community (PLC) called Subject Didactic Groups. The empirical data consists of audio recordings and documentations from meetings with one of the teacher groups as well as video recordings from student interviews and one of the teachers teaching, in the other group. The thematic analysis (Braun & Clarke, 2006) resulted in the following themes: With variation theory as a theoretical tool the teachers could a) specify what the students needed to learn, b) design and teach tasks that afforded possibilities for the students to learn what was identified as necessary to learn and c) focus on and assess qualities in the students answers. This study suggests that variation theory is a learning theory that has bearing on teachers’ teaching practice.