This master thesis is a qualitative study focusing on knowledge sharing, collegial learning and collaboration amongst teachers in an upper secondary school in Sweden. The study aims to answer the following questions: How do teachers share knowledge with each other? In which context is knowledge shared? How do teachers work towards school development? Schools in Sweden face an important decision regarding school development. One problem discussed in the thesis, raised by OECD and Skolverket, is that teachers in Sweden do not collaborate enough with their colleagues. The purpose of the study is to contribute awareness of how, when and why teachers share knowledge with each other. Five interviews were conducted with teachers in Swedish and were analysed through a thematic analysis and put in relation to knowledge sharing. The result shows that the teachers in the study share knowledge of student progression, exams, and subjects, among other things, in informal or formal groups. These are either scheduled on a weekly basis or meetings take place a couple of times each term. Furthermore, the result shows that teachers who work collaboratively can differentiate due to culture and norms at the organisation and the coworkers themselves. One conclusion is that teachers do collaborate much, but there is a difference in how depending on whether they are part of formal or informal groups. School culture determines whether or not sharing knowledge is accepted, and the school board’s value of knowledge sharing is therefore very important.