Polish accession to the European Union opened new possibilities for economic and social development. In order to seize the opportunity to accelerate the development of the country's civilization, Poland, guided by recommendations from the European Commission and the experiences of the Old Union countries, kept increasing emphasis on cooperation between the three sectors: public, social, and economic. The said cooperation, also called inter-sectoral cooperation, takes place in different areas but is mainly implemented through project activities financed by the European Fund. Creating this type of partnerships is a difficult task as it requires the close cooperation of the representatives of organisations whose priorities, objectives, and methods of operation may differ considerably. This article introduces the idea of inter-sectoral partnership based on the author's experience from the implementation of projects in Podkarpackie Voivodeship aimed at assisting people facing social exclusion.