What happens now and who do we talk to first?
April and May will see the EU attempting to agree their negotiating position as well as discussions about the “framework” for talks.
April 29 is the first Brexit summit for the remaining 27 EU member states where they will iron out any differences.
Britain’s negotiating team should first face their Brussels opponents around late May, with the Prime Minister meeting other EU leaders on June 22.
Who leads the negotiations for Britain?
Two top civil servants will be by his side. Oliver Robbins, the permanent secretary in the Brexit Department, has swiftly risen through the ranks after serving in Downing Street under New Labour.
Sir Tim Barrow has been UK's permanent representative to the EU since January, but knows Brussels well and was once our ambassador in Moscow.
Who leads the negotiations for the EU?
Michel Barnier, the former French minister. He is credited with pushing hard for the UK to pay a hefty exit bill, something that the Prime Minister has fought against.
Sabine Weyand and Didier Seeuws will be alongside him. Ms Weyand, the deputy chief negotiator with a background in trade, is widely seen as the person in charge of the detail.
Mr Seeuws, an experienced Belgian diplomat, played a key role in the negotiations surrounding the Greek bailout crisis so he has experience under pressure.
Which EU bodies are involved in the talks?
Three key EU bodies feed into the Continent’s negotiating stance – the European Commission, the European Council and the European Parliament.
Mr Barnier was appointed by the Commission, the part of the EU that makes laws. However he is given a “mandate” from the European Council, which represents the 27 different member states.
The European Parliament – the EU’s elected body made up of MEPs – can also try to shape negotiations by passing its own resolutions.
Guy Verhofstadt has the role of leading on Brexit for the Parliament, though he will not be at the negotiating table himself.
What will they actually be negotiating?
Britain wants to discuss two deals at the same time. The first is a Brexit deal, dubbed “divorce proceedings”. This is all about exactly how Britain extricates itself from the EU.
How much the UK needs to pay, what happens to EU citizens in Britain and how the Irish border will work after Brexit will all be discussed.
The second is a new free trade deal, dubbed “remarrying proceedings”. This all about what tarrifs, quotas and other agreements should be in place between the EU and the UK in the future.
Theresa May says there can be a “twin-track approach” where both can be discussed at the same time, but EU leaders insist the Brexit deal must be sorted first.
When does the British Parliament get to vote?
MPs and peers are expected to vote on the Brexit deal by late 2018. If they say no, Britain still leaves the EU but with no deal of special trading terms.
There is a more complicated process on the Continent. A Brexit deal needs to be approved by EU member states by “qualified majority”.
That means 20 of the 27 EU countries’ national parliaments must say yes, plus they must make up at least 65 per cent of the EU’s population.
Agreeing a new free trade deal is even harder. All EU members’ parliaments – including the Belgium regional parliament of Wallonia – are expected to have to say yes.
So when will Brexit be complete?
Britain will stop being a member of the EU at one minute past midnight on the 30 March 2019, by which time it is hoped an exit deal will be in place.
A “transition period” will see some EU membership benefits phased out over time. But the European Parliament said this should be done by 2022.