The article proposes an emerging approach in research on school leadership, within the framework of the "International Successful School Principalship Project (ISSPP)", where one of the three key research strands is "Principals' identities". It formulates, first, the theoretical framework for the professional identity from a narrative approach, linked--at the same time--to the practice of leadership, as an interactive relationship with the other members of the school. Successful leadership practices depend to a large degree on strong principals' identities. Second, it analyzes the biographical interviews of 15 school principals, through a process of structuring and categorizing the data collected, applying content analysis. The dimension of the principals' identities emerges in different categories: a) Personal identity; b) Professional identity (internal perspective); c) Professional identity (external perspective); d) Social identity; e) Professionalization; [and] d) Dual identity. Finally, the results are discussed, and lines are proposed to articulate and strengthen the identity of school principals in Spain.