On 23 June 2016 the United Kingdom held a nationwide referendum on whether to remain a member of the European Union or to leave. The ballot asked a single question: "Should the United Kingdom remain a member of the European Union or leave the European Union?" Voters chose between the options commonly referred to as "Remain" and "Leave." The result was 51.9% in favour of Leave and 48.1% in favour of Remain, based on a 72.2% turnout. The outcome reflected sharp regional variations: England and Wales recorded majorities for Leave, while Scotland and Northern Ireland recorded majorities for Remain. Results also varied strongly by age, education and geography, according to subsequent analyses published by the UK government and academic bodies. Politically, the vote triggered immediate consequences. Prime Minister David Cameron, who had campaigned for Remain, announced his intention to resign and left office in July 2016. Theresa May succeeded him and began the formal process of withdrawing the UK from the EU. The UK government invoked Article 50 of the Treaty on European Union in March 2017, starting a two-year negotiating period to agree terms of withdrawal and a future relationship. The referendum did not itself set out the legal or technical terms of departure; those were determined through subsequent negotiations between the UK and EU institutions. Key issues addressed in the withdrawal process included citizens' rights, the financial settlement between the UK and the EU, and the status of the land border between Northern Ireland and the Republic of Ireland. Negotiations produced a Withdrawal Agreement, which also established a transition period during which many EU rules continued to apply to the UK while the parties negotiated a future relationship. The referendum and its aftermath exposed and intensified debates within UK politics and society about sovereignty, immigration, economic policy, and the distribution of power within the UK. It also prompted renewed discussion about Scottish independence and about the constitutional settlement in Northern Ireland. Economically, markets and business decisions reacted in the immediate aftermath and across the years of negotiation; long-term assessments of the economic effects of Brexit vary and remain the subject of ongoing study. Legally and constitutionally, the referendum raised questions about the status of advisory referendums in the UK: it was held under the provisions of the European Union Referendum Act 2015 and was not legally binding in the sense that it automatically changed statutes, so parliamentary implementation was required to give effect to the result. Parliament approved the legislation to trigger Article 50 and later approved the Withdrawal Agreement and subsequent arrangements for the UK’s departure. The UK’s formal withdrawal from the EU took effect at 23:00 GMT on 31 January 2020, after which a transition period ran until 31 December 2020 while the UK and EU negotiated their future relationship. Negotiations over trade and other areas produced a Trade and Cooperation Agreement that provisionally came into effect on 1 January 2021 and was later ratified. The referendum’s political and social consequences have continued to shape UK politics and public debate. The referendum result remains a defining moment in 21st-century British history. Researchers, policymakers and commentators continue to assess its causes and consequences across politics, economics and society; where interpretations differ, those debates are noted in academic and public records.