The gap between practise and theory is often described in negative words in teachers’ professional lives. Learning study is a model for teachers´ professional development where a theoretical framework is used to enhance practice. The theoretical framework used by teachers in learning study is variation theory. In this study the research object is variation theory and the aim is to present how variation theory becomes a guiding principle to understand teaching and learning processes. Two questions are in focus in this study: In which different ways do teachers understand variation theory? Which are the critical aspects of understanding the variation theory for these teachers? Seven teachers with different experiences of learning study and variation theory have been interviewed. A fenomenographic approach was used to describe different understandings of the phenomena variation theory. To understand and to analyze the data, variation theory also has been used. As a result of the analysis three qualitative different ways in understanding the theory was found; variation is related to teaching activities, variation is related to pupils understanding and variation is related to an object of learning. Through these three categories, critical aspects of understanding variation theory were identified and these aspects point out two learning processes. The implication of this work shows that variation theory becomes a guiding principle to understand teaching and learning, when teachers relate variation to an object of learning, its critical aspects and pupils´ understanding in focus at the same time. The theory becomes a framework for analyzing and for developing teaching and learning when the theory is used in practice. When teachers use the theory in practice, they create knowledge about what is necessary for pupils´ learning. In this way practice create knowledge about the theory just as teachers are learning more about variation theory. The findings can be seen as a contribution to the discussion about theories relevance for teachers´ professional development.