The UK stopped being a member of the EU at 23:00 GMT on 31 January 2020.
What is Brexit?
Brexit - British exit - refers to the UK leaving the EU.
A public vote was held in June 2016, when 17.4 million people opted for Brexit.
the vote to leave is 52%, and for remain 48%
What happens after Brexit day?
The UK formally left the EU on 31 January 2020
This will need to be worked out during the transition period which began immediately after Brexit day and is due to end on 31 December 2020.
During this 11-month period, the UK will continue to follow all of the EU rules and its trading relationship will remain the same.
What needs to be agreed?
The transition period is meant to give both sides some breathing space while a new free trade agreement is negotiated.
This is needed because the UK will leave the single market and customs union at the end of the transition.
Prime Minister Boris Johnson insists the transition period will not be extended, but the European Commission has warned that the timetable will be extremely challenging.
What is the Brexit deal?
The transition period and other aspects of the UKs departure were agreed in a deal called the withdrawalagreement.
Most of that was negotiated by Theresa Mays government.
But after Mr Johnson replaced her in July 2019, he negotiated some changes to it.
Why did Brexit take so long?
Brexit was originally meant to happen on 29 March 2019
After MPs voted down the deal for a third time, Mrs May resigned.
This led to the new deadline of 31 January 2020.
The election, which happened on 12 December 2019, resulted in a Conservative majority of 80.