Màster en Diplomàcia i Funció Pública Internacional, Centre d'Estudis Internacionals, Universitat de Barcelona, Curs: 2016-2017, Tutora: Montserrat Pi LlorensOn the 23rd of June 2016, the United Kingdom (UK) voted to leave the European Union (EU) after forty-three years of membership. This represents an historical moment given that it is the first time that the bloc, after decades of expansion, is formally facing the loss of one of its largest members. The main objective of this paper is to explain the lengthy and complex process of Brexit from both an historical and a procedural point of view, as well as to address the possible consequences that have been put on the table after Theresa May formally triggered Article 50 of the Lisbon Treaty in late March 2017. First of all, this paper sets out that Brexit is the result of the traditional unenthusiastic and uninclined British tendency towards the European political integration project, which had its origins in the very first UK negotiations to take part in the European Economic Community (EEC) and which continued with the celebration of the first referendum on the UK membership of the European Community. Secondly, it summarises and analyses briefly the procedure for a country to withdraw from the European Union, as established by Article 50 of the Lisbon Treaty, and its interpretation by the UK Supreme Court due to the extraordinary case of the United Kingdom, a country regulated by an unwritten constitution. Particularly relevant in this regard is the fact that Brexit has raised the very significant issue of the limits of the executive powers in Britain. Moreover, the fact that Theresa May officially invoked Article 50 was soon followed by the response of the European Union, providing its guidelines and core principles for negotiations. Thirdly, some of the most significant expected effects of Brexit on the economy, the law, politics and institutions will be outlined. This paper looks into the steps taken, or to be taken, both by the United Kingdom and by the European Union and into how matters may evolve in the following time. It provides, in broad terms, the possibilities for a future association between the Union and its departing member and, finally, the consequences that the departure of a large Member State like the United Kingdom may entail for the rest of the policies of the Union and for the Union itself. At last, the methodology that will be used in order to do so is based on a set of resources, including articles, books, legislation, case law, conclusions of the European Council, EU legal acts, legislation, political statements, UK political manifestos, press sources, statistical data, white papers of both the United Kingdom and the European Union, the opinion of some of the most relevant experts in Brexit and Internet sources.