BRUSSELS (AP) -- The talks on Britain's exit from the European Union finally started Monday when EU negotiator Michel Barnier said "Welcome David" to his counterpart, David Davis, and led him toward a huge oval table at the European Commission headquarters.
As the negotiations kick off, here's a look at some of the major issues the sides face.
They will first have to unravel the British from the EU, which will be challenging to say the least. That will involve everything from deciding what waters each side can fish in to how nuclear agreements should be renegotiated. Only when there is "sufficient progress" does the EU want to look at creating a new relationship with Britain on things like trade and migration. Britain hopes the two themes - divorce terms and future relationship - can be discussed in parallel.
WHAT DIVORCE ISSUES GET TACKLED FIRST?
While Britain has struggled to agree on and present a coherent list of demands, the 27 EU nations have had one message all along - in the words of Barnier on Monday: "We must first tackle the uncertainties caused by Brexit." It means clarifying the fate of EU citizens in Britain and vice versa, how to manage the border between Ireland and the U.K., and how much Britain will pay.
MONEY, MONEY, MONEY
The EU says Britain can't leave without settling its bill, paying up for all its commitments that are still ongoing, including projects that might reach into the next decade, as well as the U.K.'s share of EU staff pensions. EU officials have put the figure at around 50 billion euros ($63 billion) while other estimates by think tanks and in the media go as high as twice that amount. As in any divorce, count on both sides to be picky in splitting the goods and dues.
WHAT ARE THE RED LINES?
The EU says it will not compromise on its core "four freedoms": free movement of goods, capital, services and workers. Britain insists that it must regain the right to control immigration and end free movement from other EU countries into Britain. May says Britain will leave the EU's single market in goods and services and its tariff-free customs union, but nonetheless, somehow, wants "frictionless" free trade.
DEAL OR NO DEAL?
Even though May triggered the two-year process on March 29, negotiators will have to get a full agreement much faster than March 2019. EU nations and the European Parliament will have to approve any future deal and that can take months. EU officials have therefore put the realistic deadline at October - and at the latest November - of 2018. If no deal is struck by then, the sides may have to create a transitional deal, possibly prolonging some of the current relationship.
If Britain crashes out of the EU without a deal, that would create huge uncertainties for citizens and businesses as well as issues like global security. How bad that would be in reality is anyone's guess.