The aim of this review in the form of a realist synthesis is to understand what makes peer (student) teacher learning activities effective. Three types of peer learning were explored: collaborative, coaching, and assessment activities. Hypotheses were developed and tested against a set of 63 studies. These hypotheses indicated what mechanisms (i.e., characteristics of the activities) would influence the effectiveness of the activities and which contexts (i.e., factors) influenced this process. Findings showed that activities wherein participants gain subject matter and pedagogical content knowledge and apply this into practice made such learning effective. Peer learning is also reinforced by facilitators, reflection, and feedback, and influenced by personal and interpersonal factors. The main contribution of this realist synthesis is the practical implications for developers of peer learning activities, for school leaders and teacher educators, and for (student) teachers.