Forty-seven years ago, in 1973, the UK joined an organisation that aimed to bring European nations closer together.
The British people voted two years later to remain in what became the European Union (EU), but in a 2016 referendum voted to leave.
On Friday 31 January, at 23:00 GMT, the UK made history as the first EU member state to legally leave the bloc.
Europeans from several of the remaining 27 member states have shared their thoughts about Brexit and the EU.
'I have mixed feelings'
Justyna Grudzien, 20, violinist from Poland
As a member of the European Union Youth Orchestra, I've met several people from the UK - and they're all concerned about Brexit.
For them, it means they can no longer be members of the orchestra, whose main goal is to connect people from EU countries.
This orchestra has over 40 years of history and from the beginning people from the UK were a very important part. I think most of the organisers are British.
For the rest of the orchestra, including me, travelling to the UK will not be as easy. We previously only needed a form of ID, but now who knows what will happen - we probably won't be able to travel without a visa.
It is heart-breaking.
On the other hand, I think the EU can, in some cases, put too much pressure on member states. The union is a wonderful idea for connecting European countries, but right now it doesn't seem to work.
'Uncertainty of no-deal still worries me'
Gabriele Caredda, 27, NHS nurse from Sardinia, Italy
As an NHS staff member, I was advised to apply for pre-settled status as it would, in theory, be enough for me to continue living and working in the UK.
The uncertainty of a no-deal Brexit worries me, though, as we don't know if the situation will change, or what will be required of EU citizens in order for us to continue working in the country in the future.
I think UK membership of the EU has been positive for both parties. It has allowed the free flow of people and ideas, and enabled a lot of young people to have experiences abroad, to face and explore a different culture and way of thinking.
I was really disappointed with the Brexit result because I always thought the UK was a kind of multicultural oasis, where everybody could find their place in a society that appreciated them for what they were bringing to the table.
'It won't have a huge impact on our business'
Kate O'Sullivan, 27, Irelands Eye Knitwear, Ireland
Brexit will have some affect on our business, but not a huge impact.
As a company based out of the island of Ireland, we have always relied on export markets to grow our business, and approximately 55% of what we produce is exported to over 20 countries worldwide.
Our exports to the UK make up about 3% of our overall business - so quite a small portion. To lose this business would be a pity, however we are in a better position than some other Irish companies who rely heavily on their UK customers.
In preparation for Brexit, we are focusing more on other export markets such as Germany, France, and the Netherlands in order to achieve growth there, although it is difficult to prepare when we do not know the exact outcomes.
The EU has been very good for Europe - the facilitation of trade agreements, the opportunities to do business and the facilitation of trade routes have all been extremely helpful, where Ireland is concerned in particular.
'It could make the EU more united'
Tudor Blaj, 43, tour operator in Romania
I have no idea what will happen with Brexit - free movement of people is my main concern. I could have fewer British clients, so it may get more difficult for me. About 60% of my clients are British.
UK membership of the EU and all the European construction has been a good thing. The EU has meant European states getting together to survive economic pressure from the US and China etc. Having a bigger market helps us compete with those big powers.
I don't think Brexit will lead to a break-up of the EU. It could make the EU more united as they will examine what made the UK leave. There was a lack of communication about the EU's goals.
'Nobody has ever left, it raises fundamental questions'
Birte Wassenberg, 52, professor in contemporary history, Strasbourg
Brexit is a kind of revolution in what it means to be in the EU. Nobody has ever left before, so this raises fundamental questions.
It may get more difficult to invite British academics to Strasbourg to talk.
UK membership has been good for Europe and the British were doing really well in the EU. They were essential for the common defence policy - without them it's a big problem. They helped the EU to develop real political power, not only economic.
A lot of French people are glad that the British are leaving - they felt the British never really belonged. But Germans really regret the UK leaving.
Having witnessed the mess of leaving the EU, others will be dissuaded from leaving too.
'I don't think the British will abandon Dutch flowers'
Edwin van Gils, 50, florist, the Netherlands
My granddad was a florist, then my father, and now it's me running this company exporting flowers to more than 1,000 supermarkets in the UK.
We've been preparing for about a year and a half. We're in contact with transport companies in the UK. It's like a long-running soap opera that started in 2016 - now it's 2020 and nothing has changed.
We are not looking at other markets. The UK is the market we know. We have a similar business sense and sense of humour. In five or six years I think things will settle down. I don't think the British will abandon Dutch flowers.
A lot of countries that were considering leaving now see how much work it is and what it will cost. When you see the uncertainty in the UK you think twice.
Interviews by Bruno Boelpaep, Mal Siret, Anna Holligan and Laurence Peter.