This thesis contributes to the establishment of a quality competency framework for the introduction of a systematic pre-service and in-service training of school principals, and emphasises the importance of professionalising the role of school principals for enhancing the quality of educational institutions within the Republic of Croatia. In order to emphasise the the importance of professionalising the role of school principals in the Republic of Croatia, the thesis first provides a detailed overview of the current literature that has contributed to both the understanding and establishment of a professionalised school principal role within a modern educational context, and emphasised the importance of professionalising their role. We argue that the existence of precisely defined competency standards is an integral part of professionalising the role of school principals, hence the thesis provides an overview of several scientific debates describing the competencies of successful school principals, and emphasising the importance of forming quality competency standards. Furthermore, the thesis provides an overview of the structure and content of school principal competency standards in the United States of America, Australia, Canada, England, and Slovenia. In addition, the thesis describes a second important segment of professionalising the role of school principals - pre-service and in-service training of school principals. We emphasise onto the importance and need for pre-service and in-service training, and provide an overview of existing approaches to pre-service and in-service school principal training within several countries.