Between Thursday and Sunday, millions will choose who represents them at the European Parliament (EP), the European Union's (EU) parliamentary institution directly elected by citizens aged 18 or older.
The EU is often considered an obscure and technocratic body - somewhere between a nation-state and an international organisation such as the United Nations. Many have lost faith in its ability to improve their lives.
But with Brexit looming and nationalist populism on the rise, this year's election - a political event held every five years - is seen by many as decisive for its future.
Britain and the Netherlands kick off voting on Thursday.
Here is what you need to know:
What is European Parliament and how will elections work?
The EP is effectively the lower house of the EU's legislative branch. It can't propose legislation - that's the European Commission's prerogative - or decide on a budget like a regular parliament. But all EU laws must be approved by a majority of EP members (MEPs) to then be applied in all 28 member states.
Originally, the EP's powers were limited. But as the EU grew and its member states shared more responsibilities, the EP became co-legislator with the European Council.
Elections are held at the national level and respect each country's electoral tradition. The British and the Dutch will be first to vote on May 23, followed by the Irish on May 24, and the Maltese, Latvians and the Slovaks on May 25. Most countries will vote on May 26, a Sunday. The minimum age to vote is 18, except in Austria and Malta where it's 16.
There are 751 seats, but this number will go down to 705 after Britain exits the bloc.
Each member state is allocated a certain number of representatives, or MEPs, depending on their total population.
Germany, the most populous EU member, will send 96 MEPs to Brussels, whereas Malta and Luxembourg only get to elect six.
Every country votes on the principle of proportional representation, meaning that each political party or list of candidates gets a number of seats equivalent to their share of the vote.
A party needs to gather at least 5% of the votes in one country to enter the European Parliament (EP).
There are eight pan-European political groups that MEPs can choose to join.
It takes 25 MEPs from seven different countries to form a political group.
The two largest groups are the centre-right European People Party (EPP) and the centre-left Socialists and Democrats (S&D). The right-wing eurosceptic European Conservatives and Reformists (ECR) is the third largest.
Current EP President Antonio Tajani, an Italian, is with the EPP.
What's expected to happen?
For the first time since 1979, the centre-left and the centre-right could lose their majority and wave goodbye to their ruling coalition.
"The centre-left will recede again, the centre-right should minimise their losses, and the far right will continue to rise. But the big question is: what will happen at the centre?," said Amandine Crespy, a professor of European studies at the Universite Libre de Bruxelles.
In terms of voter turnout, between 1979 and 2014, it fell from 62% to only 42%. This year's projections don't look optimistic.
Why is the 2019 vote important?
There are two factors. The first is that the vote takes place amid Brexit, which marks the first time a country will leave the EU.
"There is now a feeling of urgency, Europe is going through an existential crisis," said Crespy.
The second is that the vote could also be divisive because of an expected rise in anti-EU forces, or Eurosceptics.
"Those parties oppose the principle of a democratic EU and want to change it from within," said Sylvain Kahn, a professor of European studies at Science Po Paris.
According to Shahin Vallee, a researcher at the European Institute of LSE, "They will set the tone of the European debates for the next five years.
"In May, we will be able to tell what direction the EU will take on a whole number of issues, from the regulation of the GAFAs (giant tech firms) to its attitude towards China, and of course, its migration policy."
How will Brexit affect the vote?
The UK was due to leave the EU on March 29, 2019. But it didn't. The EU Council decided that if the UK doesn't leave before May 22, it should take part in the European elections like any other member state.
"It will turn into a second referendum," said Crespy, "creating an impossible situation in the EP and other European institutions".
Until the UK leaves, it remains a rightful EU member with its EU commissioner, voting rights in the EU Council, and its 73 MEPs, including far-right Nigel Farage - chair of the populist Europe of Freedom and Direct Democracy political group.
Vallee fears the UK's participation in the vote would lead to a "one foot in, one foot out [situation] which would most certainly paralyse the already fragile functioning of the union's institutions".
Will the elections affect the EU's influence on the world stage?
Europe is often seen as an economic giant but political dwarf. The EU remains a political structure in the making.
"Europe's weakness is first and foremost political," said Vallee.
Between 2012 and 2014, during the Eurozone crisis, Vallee was an adviser to then-president of the European Council, Herman Van Rompuy.
"The European Union suffers from two evils," he said. "It doesn't have an identified government, able to make strategic decisions like the United States or China. And it lacks democratic legitimacy because the EP is too weak."
Since the creation of the EU in the 1950s, a fierce debate on the notion of sovereignty has agitated the continent.
"The big question is: Will we be more efficient and resilient to cope with world's challenges by uniting with other countries or not? One for all and all for one like the musketeers or by yourself?" said Kahn.
Vallee advocates for radical change.
"We need to create European sovereignty. Sovereignty is to be understood as the capacity to act on the world's course; therefore we need a real European government and parliament, independent from the member states."