The research question addressed in this paper is: How do the activities of writing mediate knowledge of writing, disciplinary knowledge, and professional knowledge as intertwined sites of learning? To conceptualise the role that writing can take in these complex processes, we apply an analytical framework comprising two core concepts; mediation and learning trajectories. We draw on an empirical study from the context of initial teacher education in Norway. From our analysis, we identify three qualities of writing as important. First, the (...) writing process should in- clude responding to and sharing drafts. Other important qualities include high teacher expectations and continuous reflection. From the perspective adopted here, learning is understood to be distributed and situated. In particular, in situated cultural contexts, collaborative writing can become a significant mediational tool for learning. Initial teacher education seeks to prepare the student teacher for a highly complex professional competency, developing both professionally and in individual subjects. To do so, students must transform social structures and the tools embedded in practices into psychological tools. We contend that writing is one significant tool in moving through complex trajectories of learning towards becoming professional teachers. (shrink)