Britons voted in a referendum on Thursday to leave the European Union. Following are answers to key questions on what will happen next in Britain's relations with the bloc:
1. WHAT DOES IT ALL MEAN?
The EU is in shock and entering uncharted territory. No member state has ever left and Article 50 of the EU treaty, which sets out how a state can exit the bloc, offers little detail. Although it provides a sketchy legal framework for a two-year period of withdrawal, many fear the process can quickly become acrimonious, disrupting the economy and European affairs across the board.
Cameron has said he will notify the Union "immediately" that Britain is leaving by invoking Article 50. But it is not clear how quickly Britain will set that two-year clock ticking and the EU itself cannot, officials believe, trigger the process itself.
Some Brexit campaigners have suggested Britain should wait before triggering Article 50 to give more time for negotiation, possibly even to win better EU membership terms or to secure a deal to retain British access to EU markets once it has left.
EU leaders have ruled out further talks on membership -- "Leave means leave," they say -- and many want a quick, two-year divorce while negotiating terms for a future, arms-length relationship may take much longer.
However, major EU powers appear keen to see as orderly a transition as possible to a new relationship. That could involve Article 50 negotiations being extended beyond two years to allow time for a broader deal. But such an extension requires the consent of all 28 member states, and reaching that unanimity could be problematic.
A deal Cameron struck with EU leaders in February to curb immigration, protect London finance interests from the euro zone and opt out of "ever closer union" is killed by the referendum.
If no treaty is agreed, EU law simply ceases to apply to Britain two years after it gives formal notice it is leaving.
Until a departure treaty is signed -- which requires assent from Britain and a majority of the remaining 27 states weighted by population -- Britain remains, in principle, a full member of the EU but will be excluded from discussions affecting its exit terms. In practice, many expect British ministers and lawmakers to be rapidly frozen out of much of the Union's affairs.
Some Brexit campaigners have also said Britain should act more quickly, for example to stop funding the EU budget or curb immigration from EU states. That could provoke EU reprisals.
An array of laws and EU entitlements will cease to apply to British business and citizens, creating what Brexit campaigners say will be opportunities for more growth and more selective immigration, but which Cameron has said will do long-term damage to the economy and Britain's global influence.
2. WHERE DOES THE EU GO FROM HERE?
The Union needs quickly to fill a 7-billion-euro hole in its 145-billion-euro annual budget, which is currently fixed out to 2020, as it loses Britain's contributions while saving on what Britons receive from EU accounts.
The EU will also want to clarify as quickly as possible the status of firms and individuals currently using their EU rights to trade, work and live on either side of a new UK-EU frontier.
Britain is likely give up its six-month presidency of EU ministerial councils, due to start in July next year. Its place may be filled by Estonia or, possibly Malta or Croatia.
EU leaders may push for a quick show of unity on holding the bloc together in the face of eurosceptics inspired by the result in Britain -- including National Front leader Marine Le Pen, who leads polls for next April's French presidential election.
Divisions between Berlin and Paris on managing the euro zone probably rule out a big move on that front before both hold elections in 2017. Closer EU defence cooperation, without sceptical Britain, may be revived. A major EU security policy review is already on the summit agenda as is a new push to tighten control on irregular immigration from Africa.
Many leaders caution against alienating voters by moving too fast on integration, which they say has alienated voters. Summit chair Tusk wants to launch a formal process of reflection on where the Union has failed to connect with people.
3. SO WHAT CHANGES?
In principle, nothing changes immediately. Britons remain EU citizens and business continues as before. In practice, many believe trade, investment and political decisions will quickly anticipate British departure from the bloc. The EU could also face a Britain breaking apart if europhile Scots make another push for independence and seek to join the EU on their own.
There is a "Brussels consensus" that Britain must be made an example of for leaving and will face a chilly future, cast out to perhaps talk its way back later into some kind of trade access in return for concessions such as free migration from inside the bloc and contributions to the EU budget - things which Brexit voters want to end but which the likes of Norway and Switzerland have accepted in varying forms.
However, cautious diplomats do not rule out surprise turns.