On the basis of the theoretical analysis of the in-service training model for the training of school principals in the area of school management, we obtained an overview of insights, findings, different approaches and practitioners’ experiences belonging to individual “schools”, authors and countries. The analysis has provided relevant conclusions about the trends in contemporary in- service training of school principals, the groups of existing models according to years of work experience of principals, and the aims of in-service training of secondary-school principals. The models that appear to be the most important and the most widespread are the models of in-service training that aim at producing transactional and transformational school principal, the school principal who leads by “serving”. The two models that prevail on the global scale are mentored leadership and the model of modular in-service training. Depending on the number of years of work experience of school principals, the models are divided into three core groups: models of pre- service training, models of in-service training for beginning principals in the first year(s) of service, and the continuous professional development of experienced school principals. Empirical analysis of the model of in-service training of secondary-school principals for school management, which is currently implemented in Croatia, has shown that the school principals, who took part in the survey, attach the most importance to all in-service training models for the training of school principals in the area of school management, and the in-service training for school principals in the first year(s) of service. The lack of mentored leadership was identified as the biggest weakness of the current model by the principals of Croatian secondary schools who participated in the survey. In their opinion, this is the most important model to be implemented at the very beginning, and during the first years of service. The school principals with less work experience attached more importance to in-service training in school management, then their more experienced colleagues. In designing a hypothetical model of in-service training of secondary-school principals in Croatia, the positive results of different countries were used for the defining and implementation of certain models presented in the theoretical part of the thesis. The models or parts of models that were analysed were the ones whose development and practical application was imbedded in the context, and dealt with problems, similar to the ones encountered in Croatia. Special attention was given to the key elements of the models that are easily recognizable and applicable in the context of in- service training of secondary-school principals in Croatia. Hereby we specifically refer to the issue of the selection procedure of secondary-school principals in Croatia which does not envisage obligatory pre-service training for candidates for school principals. Financial justification of introducing the new hypothetical model of in-service training of Croatian secondary-school principals in school management has also been analysed. The comparison of the analysis of actual costs of annual in-service training of secondary-school principals in school management in the current in-service training system, with the simulation of annual costs of the hypothetical model, shows that the introduction of the new hypothetical model is financially justifiable.