Leadership development has gained great interest internationally. There are a variety of approaches in leadership development of school leaders. Leadership development ininterprofessional settings is among the trends. This thesis examines how leadership development evolves in an interprofessional team. The interprofessional team was situated ina Norwegian context, and consisted of principals, administrators and researchers. The team was formed to support the principals in three schools in leading a local school improvement project aimed at increasing students’ expertise of approaching academic texts in different subjects with specific learning strategies. The study is based on artifacts, audio and video data. In particular the study examines how school data trigger and structure the departure point for professional development in an interprofessional team. The study also examines how and to what extent professionals from schools, a municipality, and a university develop a shared understanding of the collaboration. In addition the study examines how leadership development evolves when principals, administrators, and researchers jointly analyze different challenges in a school improvement project over two years of collaboration. Third generation Cultural Historical Activity Theory (CHAT) is chosen as a point of departure for creating an analytic framework for the study. The study includes three sub-studies. The overall findings suggest that how leadership development evolves can be characterized as: a struggle with ill-defined purposes of the collaboration; horizontal dynamics across professions and working contexts; explorative work, mediated and structured by tools. The findings also suggest that the dimensions of time and space influenced the boundary work in the team. The implication of the study is to pay attention to the role of tools, tensions and any ill-defined purposes when adjusting programs and designing opportunities for school leaders to develop their expertise in the future