In order to understand leadership, it is not enough to assume that organizations act as rational instruments. We need a better understanding of the dynamics of schools. Researchers have recently found that leadership's dynamism has a different meaning if we see organizations as complex systems and leaders in interacting and influencing. The purpose of this article is to discuss what leadership research has shown and what conclusions can be drawn. The starting point is research reviews, and evidence-based leadership, efficiency, problem-solving and methodology, leadership in complex organizations, practice-oriented leadership research and the relationship between leadership and learning. The article concludes, among other things, with a reflection on the school's scientific basis. One possibility may be to try to develop a "science" within the framework of their own activities in school and classrooms. Principals leadership is developed in practice and with inspiration from research, especially research that is practice-oriented. As shown in this article, the research provides different messages about what is successful. But we also need to use the studies that provide a deeper understanding of leadership dynamics and leadership aspects that have not been studied sufficiently.