The aim of the article is to explore newly appointed principals' professional identity formation in Sweden. By addressing five specific aspects of principals' professional identity, the article analyses how principals depict their 'knowledge', 'understandings', 'experiences and wisdom', 'values' and 'beliefs' in forming a professional identity. Another focus is on newly appointed principals' knowledge, understandings, experiences and wisdom, values and beliefs, given their biographical histories. The empirical analysis rests on a qualitative survey conducted with 87 principals who were about to begin the National Principal Training Programme that all newly appointed principals in Sweden have to follow. Theoretically, the analytical work rests on a biographical perspective on principals' professional identity. The main results show that the identity of newly appointed principals is diverse and also fragmented. Moreover, from a biographical point of view, there are facts prior to professional experience-knowledge that are significant in identity construction and that give meaning to an identity as school principals.