Why has it taken so long?
Britons voted by 52 percent to 48 percent to leave the EU in a referendum back in June 2016, but the UK government couldn't cut the cord from the 28-member European bloc until it had the approval of its parliament.
In between, there was the matter of Prime Minister David Cameron's stepping down from his post and Theresa May taking over.
Parliament approved the Brexit bill on March 13, which paved the way for May to trigger Article 50 and begin formal proceedings Wednesday to leave the EU.
What is Article 50?
Also known as the "withdrawal clause," Article 50 was introduced in the EU's Lisbon Treaty signed in 2007, and brought into force in 2009.
"A member state which decides to withdraw shall notify the European Council of its intention," it reads.
May triggered Article 50 on Wednesday, formally informing the European Council of its wish to leave the bloc after 44 years.
"This is an historic moment from which there can be no turning back," she told a noisy session of parliament in an address that was frequently interrupted by heckling from opposition lawmakers.
But the "divorce" proceedings will not be simple.
So Britain hasn't really left the EU yet?
Symbolically, Britain is now on their own. And that's about it.
Two years of formal negotiations will now commence. These talks between both parties will establish future ties between Britain and the EU, but are not expected for several weeks while the EU formalises its position.
Many in Britain would like to see a transition period put in place where existing EU legislation will continue to apply even after Britain has left.
But the EU has already signalled that it would be "difficult to imagine" an interim deal before the terms of the future relations with the UK are set.
A full deal for the future relationship will probably take years, up to seven, according to European Council President Donald Tusk, or even a decade, according to reported comments made by Britain's former ambassador to the EU.
What are the key issues that need to be resolved?
The fate of EU immigrants in Britain and Brits in the EU is the main issue. Security, trade and Britain's outstanding "Brexit bill" are others.
Both sides have said they would like an early resolution on the status of more than three million Europeans living in Britain, and more than one million Britons living elsewhere in the EU.
May said she will prioritise reducing the hundreds of thousands of immigrants from the rest of the bloc who move to Britain each year. It was an issue that dominated the referendum campaign.
Britain's exit from the EU will cost it close to $63 billion, European Commission President Jean-Claude Juncker said earlier this month. This amount is part of Britain's payment to the EU budget which finances infrastructure projects, social programmes, scientific research and pensions for EU bureaucrats.
May is also pushing for "maximum possible access" for British companies, but without a new trade agreement, Britain would fall back on World Trade Organisation (WTO) rules, which could mean higher export tariffs and other barriers.