Here is an easy-to-understand guide to Brexit - beginning with the basics, then a look at the negotiations, followed by a selection of answers to questions we've been sent.
What does Brexit mean?
It is a word that is used as a shorthand way of saying the UK leaving the EU - merging the words Britain and exit to get Brexit, in the same way as a possible Greek exit from the euro was dubbed Grexit in the past. Further reading: The rise of the word Brexit
Why is Britain leaving the European Union?
A referendum - a vote in which everyone (or nearly everyone) of voting age can take part - was held on Thursday 23 June, 2016, to decide whether the UK should leave or remain in the European Union. Leave won by 51.9% to 48.1%. The referendum turnout was 71.8%, with more than 30 million people voting.
What was the breakdown across the UK?
England voted for Brexit, by 53.4% to 46.6%. Wales also voted for Brexit, with Leave getting 52.5% of the vote and Remain 47.5%. Scotland and Northern Ireland both backed staying in the EU. Scotland backed Remain by 62% to 38%, while 55.8% in Northern Ireland voted Remain and 44.2% Leave. See the results in more detail.
What is the European Union?
The European Union - often known as the EU - is an economic and political partnership involving 28 European countries (click here if you want to see the full list). It began after World War Two to foster economic co-operation, with the idea that countries which trade together were more likely to avoid going to war with each other.
It has since grown to become a "single market" allowing goods and people to move around, basically as if the member states were one country. It has its own currency, the euro, which is used by 19 of the member countries, its own parliament and it now sets rules in a wide range of areas - including on the environment, transport, consumer rights and even things such as mobile phone charges. Click here for a beginners' guide to how the EU works.
When is the UK due to leave the EU?
For the UK to leave the EU it had to invoke Article 50 of the Lisbon Treaty which gives the two sides two years to agree the terms of the split. Theresa May triggered this process on 29 March, meaning the UK is scheduled to leave at 11pm UK time on Friday, 29 March 2019. It can be extended if all 28 EU members agree, but at the moment all sides are focusing on that date as being the key one, and Theresa May has now put it into British law.
So is Brexit definitely happening?
The UK government and the main UK opposition party both say Brexit will happen. There are some groups campaigning for Brexit to be halted, but the focus among the UK's elected politicians has been on what relationship the UK has with the EU after Brexit, rather than whether Brexit will happen at all. Nothing is ever certain, but as things stand Britain is leaving the European Union. There is more detail on the possible hurdles further down this guide...
What's happening now?
The UK and EU have provisionally agreed on the three "divorce" issues of how much the UK owes the EU, what happens to the Northern Ireland border and what happens to UK citizens living elsewhere in the EU and EU citizens living in the UK. Talks are now focusing on the detail of those issues - there is yet to be agreement on how to avoid having a physical Northern Ireland border - and on future relations. To buy more time, the two sides have agreed on a 21-month "transition" period to smooth the way to post-Brexit relations. The UK cabinet has agreed how it sees those future relations working and will now be seeing if the EU agrees.
What is the 'transition' period?
It refers to a period of time after 29 March, 2019, to 31 December, 2020, to get everything in place and allow businesses and others to prepare for the moment when the new post-Brexit rules between the UK and the EU begin. It also allows more time for the details of the new relationship to be fully hammered out. Free movement will continue during the transition period, as the EU wanted. The UK will be able to strike its own trade deals - although they won't be able to come into force until 1 January 2021.
Do we know how things will work in the long-term?
No. Negotiations about future relations between the UK and the EU are taking place now. Both sides hope they can agree by October on the outline of future relations on things like trade, travel and security. If all goes to plan this deal could then be given the go ahead by both sides in time for 29 March 2019. Theresa May delivered a big speech setting out her thoughts on the UK and EU's future relations on 2 March, 2018.