The doctoral thesis “Impact of Entrepreneurial Orientation on Social Innovation and Economic Sustainability of Social Entrepreneurship Organisations” aims to explore and analyse all significant concepts of social entrepreneurship. As there is no consensus on the definition of social entrepreneurship, there is no consensus on defining a social entrepreneur, which highlights the complexity of the area under investigation. The thesis addresses, from both theoretical and empirical perspectives, the issue of the impact of entrepreneurial orientation on social innovation and the economic sustainability of social entrepreneurship organisations. The empirical part of the research is based on the primary data collected through field research, a questionnaire survey and a combined method of focus groups and semi-structured in-depth interviews. The research findings confirmed that entrepreneurial orientation, which is described through three key dimensions: innovation, proactivity and risk-taking, may contribute to social innovation and economic sustainability of social entrepreneurship organisations. More specifically, the two main hypotheses of the research were confirmed, according to which (1) entrepreneurial orientation expressed by the dimensions of innovation, proactivity and risk-taking has a positive effect on the economic sustainability of a social entrepreneurial organisation, and (2) entrepreneurial orientation expressed by the dimensions of innovation, proactivity and risk-taking has a positive effect on social innovativeness. In addition, factors that determine the degree of entrepreneurial orientation in social entrepreneurship were additionally identified (socially oriented mission, perception of opportunities for entrepreneurial activity, perception of opportunities for obtaining financial resources, involvement of relevant stakeholders in the organisation's mission), while regression analysis determined that socially oriented mission and the involvement of relevant stakeholders in the organisation's mission positively affect the entrepreneurial orientation of social entrepreneurship organisations. Moreover, results depict the key mechanisms of creating social value in social entrepreneurship organisations which are relevant for both the academic and business communities and for all strategic decision-makers in social entrepreneurship organisations.