The aim of the article is to analyze the impact of membership in the European Union on Poland's energy security in the gas sector. The development of the EU energy policy as well as the EU internal market rules has had many positive effects on improving the energy security of the EU Member States, including Poland. Regulations of the EU energy law and competition rules give the Member States the opportunity to use important legal tools. In addition, the political and fi nancial support of

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... ls for the development of energy infrastructure gives the possibility of real integration with other countries within the EU energy market. On the other hand, the legal and institutional specifi city of the EU as well as differences of interests between EU Member States mean that EU membership contributes little to the diversifi cation of sources of supply, which remains one of the key objectives of Poland's energy policy.