David Cameron is in Brussels for what is likely to be his last European Council summit as UK Prime Minister.
Here are the key players at the summit who will be involved in the Brexit negotiations.
1. Jean-Claude Juncker, president of the European Commission
David Cameron tried to block the former Luxembourg PM from getting the job as the EU's top bureaucrat and then forced him to go through months renegotiating the UK's membership, so some may forgive him for relishing the PM's current discomfort.
Juncker wants talks on UK exit to begin immediately, saying: "It's not an amicable divorce, but it was not exactly an intimate affair anyway."
2. Donald Tusk, president of the European Council
The former prime minister of Poland now chairs meetings of the 28 - soon to be 27 - national heads of government of member states.
Tusk is facing calls for his resignation from Polish political rivals, who accuse him of playing a "dark role" in blocking meaningful reforms in Cameron's renegotiation.
Before the referendum he warned Brexit might lead to the end of Western civilisation, although afterwards, he said the vote was a "historic moment" but "not a moment for hysterical reactions".
3. Angela Merkel, German chancellor
As leader of the EU's biggest and richest member state, Merkel is used to getting her way in Brussels.
She is bitterly disappointed at the prospect of losing the UK, which Berlin sees as a free-market counterweight to France and the Mediterranean states.
She has said there is no need to be "particularly nasty" to the UK in Brexit negotiations.
4. Francois Hollande, president of France
Hollande is facing a challenge in upcoming elections from populist National Front leader Marine le Pen, who was one of the few European politicians to acclaim the outcome of Thursday's vote.
Fearful of the consequences of prolonged political and economic instability, he has called for a swift conclusion to Brexit talks, saying: "Being responsible means not wasting time in engaging with the question of Britain's departure and setting this new impulse we want to lend the new European Union."
5. Mark Rutte, the current holder of the European Council's six-month rotating presidency
The prime minister of the Netherlands is facing calls at home for a "Nexit" vote in his country.
He has described the Leave vote in Britain as "disappointing" and said it is important that a solution for the crisis is found "calmly and step by step".