The main purpose of this doctoral thesis is to contribute to a better understanding of the various factors and indicators associated with higher school effectiveness, as well as to provide a preliminary insight into the present state of effectiveness of primary schools in the Republic of Croatia. The thesis begins with a detailed overview and analysis of theoretical discussions and empirical research on school effectiveness in the international and national context. The analysis indicated the main characteristics of school effectiveness as well as its key factors and indicators. In this research school is defined and explained in the context of the system approach to work organization, therefore, the thesis outlines the main approaches to the conceptualization of effectiveness with special emphasis on issues related to general characteristics and functioning of effective schools. School effectiveness is defined in line with different criteria for effectiveness assessment such as productivity, adaptability, involvement, continuity, quality management, satisfaction, collaboration with the stakeholders. In accordance with the selected effectiveness criteria, a theoretical - empirical model was created as a starting point for the empirical research. Subsequently, different contextual features of schools, general school features (school inputs), school processes and performance indicators (school outputs) were operationalized. In view of the above mentioned, school effectiveness has been asessed based on different groups of indicators, i.e. on the basis of the effectiveness assessment of different aspects of school activities: Effective school performance focused on students outcomes, Effective school performance focused on pupil discipline, Effective school performance focused on evaluation, Effective school performance focused on enriching the school curriculum, Effective socio - public performance of the school, Effective school performance focused on cooperation, Effective school performance focused on employee satisfaction. In line with the established theoretical - empirical model, this doctoral thesis described the contextual and general features of primary schools in the Republic of Croatia, and then the present state of elementary schools’ effectiveness based on the selected indicators. Furthermore, it examined whether there are differences in the effectiveness of primary schools in the Republic of Croatia, given the contextual and general characteristics of the school, as well whether there are differences in the effectiveness of primary schools in the Republic of Croatia with regard to the dominant type of school leadership (selected variable of school processes). The basic unit of analysis in this research consisted of primary schools in the Republic of Croatia. The population consisted of all central primary schools in Croatia, and the research involved a sample of 56 central primary schools. The survey method was used to collect the data and the newly constructed Questionnaire for Collecting Objective Efficiency Indicators and School Characteristics was distributed. The data analysis was conducted using the Statistical Program for Social Sciences (IBM SPSS, 20.0.). The research results have shown that most of the primary schools in the Republic of Croatia involved in this research belong to a group of more effective schools, and to a lesser extent to the moderately effective school group. None of the schools in the sample belong to a group of lower effective schools. Furthermore, there is a statistically significant difference in the effectiveness of certain aspects of school performance and the overall effectiveness of primary schools with regard to the contextual and general features of schools such as founder, location, school status, school size, number of shifts in school, pupil - teacher ratio, percentage of the pupils who travel, percentage of pupils using subsidies, the number of teachers with the necessary expertise in their field, percentage of promoted teachers, forms of educational work being carried out in schools, school equipment and obtaining additional financial resources. Furthermore, there is a statistically significant difference in the effectiveness of primary schools in the Republic of Croatia with regard to the dominant type of school leadership, i.e., the schools where the distributed school leadership is more dominant are more effective in terms of employee and user satisfaction. It has also been shown that among the selected moderators (contextual and general features of the school) there are no statistically significant moderators on the significance of differences between schools with different dominant types of school management and school performance indicators. The results of the conducted research indicate the need to improve the level of effectiveness of Croatian primary schools by enhancing all observed aspects of their work, especially those which indicate a lower level of effectiveness (regarding each observed school). This primarily entails empowering the principals and other key stakeholders involved in the educational process so as to better understand the underlying factors that contribute to greater school effectiveness, to recognize and understand the key performance indicators, and ultimately to address the most important aspects of school work contributing to higher school effectiveness. Research results also point to the necessity of equalizing the working conditions in Croatian primary schools, especially with regard to providing sufficient financial resources for the functioning of the school in all aspects. This should be done through providing better support to schools by the founders and ensuring the basic infrastructural and human prerequisites for the educational process. The obtained research results provided a basis for development of guidelines and recommendations for researchers and participants in the educational process as well as the competent institutions and policy makers at the national level.