The Treaty of Rome (hereinafter referred to as: EEC Treaty) establishing the European Economic Community (1957) determined the main objective of the European integration project as intending to establish “an ever closer union among the peoples of Europe.” In accordance with this paradigm, the founding Treaties did not include any provisions on the exit of a member state. First, the European Constitutional Treaty included the unilateral right to withdraw from the European Union (hereinafter referred to as EU), however, due to the referenda in France and the Netherlands, it never came into force. Thus, European Community (and later EU) Law did not regulate this issue, until the Lisbon Treaty came into force in 2009. Although the question of a Member State’s unilateral withdrawal was raised up several times during the history of the European Union, no member state has withdrawn from the EU yet. The referendum of 23 June 2016 on EU membership in the United Kingdom (hereinafter referred to as: UK) resulted in an unprecedented and surprising situation, when the first application of Article 50 of the Treaty of the European Union is in process. This article aims to summarize the main legal aspects of the withdrawal.