Projects of common interest are projects serving the implementation of priority corridors and areas of energy infrastructure, mainly trans-European electricity, gas and oil transmission networks in order to reduce the European Union’s energy dependence. They also help strengthen the energy security of individual countries, including Poland. The article aims at reviewing projects and assessing an impact on domestic markets in the context of the challenges of the fuel and energy sector in Poland. The article is an analytical study, in the “desk research” formula. Research methods used during the elaboration of the research include the analysis of national and EU strategic documents and the subject matter of energy security. In order to evaluate the impact of the implementation of projects of common interest on a variety of supplies of a member country (Poland), an exemplary assessment of the diversification status is presented using Hirschman-Herfindahl concentration index for gas supplies.