It is the second largest democratic contest on the planet, as hundreds of millions of voters go to the polls to elect over 750 MEPs.
From hard right to radical left, every political hue is represented, with a staggering 15,500 candidates taking part.
Here is what you need to know about the European elections:
More than 360 million EU citizens are eligible to vote in the European elections. 751 MEPs will be elected, including 11 in Ireland.
Over the years, as the EU has enlarged so has the membership of the European Parliament.
The reverse will apply if and when the UK leaves.
While the total number of MEPs will be reduced to 705, some of the UK's 73 seats will be redistributed to other states, including Ireland, whose number will actually rise to 13 after Brexit.
France and Spain would gain the most from the redistribution, with each getting an additional five MEPs.
This is not only the first election where the EU is faced with the imminent prospect of losing a member, it is also the first European election in a quarter of a century where the EU has not increased in size since the previous ballot.
The world's largest democratic contest is the Indian election. The European elections get the silver medal.
The parliament, which turns 40 this year, is the only directly elected EU institution. These are its ninth elections, with MEPs serving a five-year term.
MEPs vote on European legislation, which is binding across the EU, and shapes everything from hours worked per week, to the use of pesticides.
While the parliament was somewhat sidelined in the aftermath of the financial crisis, it does approve each of the European Commissioners and, more importantly, the Commission president (the European Commission is, basically, the EU's civil service).
Since the Lisbon Treaty came into force ten years ago, the European Parliament has also overseen the allocation of the budget (but not its size, which is set by the Commission).
According to EU research, the issues of greatest concern for voters are firstly, immigration, which, while declining in importance in voters' minds, it is still by far the hottest topic. Secondly, the economy, which is closely followed by climate change which has been steadily increasing as an issue for the past decade. And then, crime.
The parliament's official seat is Strasbourg, where MEPs meet monthly for plenary sessions lasting four days.
The rest of the time they meet in Brussels, 440km away (for some, this arrangement is a source of exasperation, as it is estimated to cost over €100m per annum. Any adjustment would require treaty changes, and so is unlikely).
Both the voting age and a candidate's minimum age are established by national law.
The voting age is 18, except in Greece where it is 17, and in Austria and Malta where people can vote fromt he age of 16.
However, the minimum age for a candidate varies from 18-years-old in the vast majority of states to 25 in Italy and Greece.
Voting is compulsory in five states: Belgium, Bulgaria, Cyprus, Greece and Luxembourg.
Registered non-national EU citizens are also required to vote.
Voting systems vary between states, with some voting for parties, and others for candidates. EU rules require that a form of proportional representation is used.
Unlike the 'first past the post' UK system, PR means parties gain seats in proportion to the amount of votes they get, and is generally thought to benefit smaller parties.
Most countries use some variation of the "list" system, with only Ireland and Northern Ireland offering a single transferable vote. In some countries, including Ireland, MEPs represent regions, while in others, such as Germany, each MEP represents the entire nation.
Dutch and British voters were the first to cast their ballots yesterday.
Ireland is the only country to vote today, followed by a handful on tomorrow (Latvia, Malta and Slovakia; the Czech Republic votes on both days) but the vast majority of EU citizens (21 states) will go to the polls on Sunday.
Results cannot be announced before the last polling station - in Italy - closes on Sunday at 10pm Irish time. But by that stage we will have exit polls and tallies, and should have a reasonable indication of how things are looking for the candidates in Ireland.
National parties and independent candidates contest the European elections.
Once elected, MEPs join parliamentary blocks which are transnational groups according to political ideology.
There are eight political groupings in the outgoing parliament (some with rather convoluted names) ranging from the far-right Europe of Nations and Freedom, to the radical European United Left-Nordic Green Left.
The largest political grouping plays a decisive role in choosing the President of the European Commission, who is selected by the European Council (made up of the leaders of EU nations), but is approved by parliament.
The European Peoples Party is currently the largest grouping. It is home to Fine Gael, Angela Merkel's CDU and Commission President, Jean-Claude Juncker.
Turnout has fallen at each and every EU election since the first one in 1979, when over 60% of those eligible cast a ballot.
However, over the past decade that slump has generally slowed, but not in Ireland, where turnout fell from 59% in 2009 to 53% in 2014.
According to official EU figures, overall turnout in 2014 was just over 40%, and while it pushed 90% in Belgium (where, as we have noted, voting is obligatory) it was only 13% in Slovakia (where turnout in European elections has never reached 20%. Polish turnout has yet to reach 25%).