The article concerns primarily the effects of the membership of the European Union on national (Polish) law and, to a limited extent, on the political system of a state. The conclusions presented in the article are of universal value. Although the article deals with Polish affairs, the principles, tendencies and consequences identified are typical of the relationship state – the EU, both before and after accession, regardless of the state concerned. It should be, however, noted that the path to membership and the membership itself are different in each case. The practice of the Polish membership of the European Union, its systemic dimension and the changes in the national legal system (Europeanisation) do not differ significantly than in the case of other Member States. Europeanisation of Polish law, politics, economy, culture and society has been in progress since the 1990s. One can differentiate between two stages of Europeanisation: before and after Poland’s EU accession, each characterised by different conditions. Over time, this process, on the whole, has been undergoing numerous changes but it has never weakened in importance. Poland faces issues such as poor legitimation of integration processes, supremacy of the government over the parliament, passivity of parliamentary committees in controlling the government and EU institutions in the decision making process, as well as dilution of responsibility for decisions taken within the EU. The process of Europeanisation relies mostly on direct application of the standards of EU law in the national legal system, implementation of directives into national law and harmonisation or standardisation of national legal solutions so that they comply with the EU framework. It is also reception of a common, European (Union) axiology.