Professional Learning Communities– A Necessity for Improving Teaching? The aim of this study was to examine what factors are important for teachers in professional learning communities to improve their teaching in order to increase the results of the students. The study was based on a program of development about language improvement that took place in two schools during the autumn of 2014. The authors of this study were also in charge of the improvement program. The data used in the analysis was based on individual questionnaires and group interviews. The theoretical framework of the study was founded on Timperley’s (2011) inquiry and knowledge-building cycle, Hargreaves and Fullan’s (2012) theory about professional capital and Stoll’s (2006) characteristics of effective learning groups. The results obtained in this study indicate that the following factors are of crucial importance for teachers to improve their teaching; leadership, systematic surveys of what students need to develop, a clearly understandable aim within the specific area of development, a clear structure within the learning group and between the meetings, scheduled time for the meetings, teachers’ mindset, a scientific approach, the use of science and initiated moderators of the learning groups. The study showed that professional learning communities working with these factors are going to be successful.