Tale of the tape: What are the parties hoping for?
The results of the European Parliament elections will be pored over as a barometer of public opinion ahead of next year's general election. What are the parties hoping for?
The UK Independence Party
The hype around UKIP's performance in these elections has been bubbling away for months. The party leader Nigel Farage promised an "earthquake" in British politics and has said, on and off, for months, that UKIP will win more seats in the European Parliament than any other British party. The thing is, success in politics, as in so many things, is often measured as the gap between what is expected and what is achieved.
But UKIP hasn't really gone in for the expectation management other parties so often indulge in - talking down their prospects so the eventual result looks better by comparison. In short, anything less than winning more seats than any other British party will be seen by UKIP's critics as a failure.
After all, they came second last time in 2009, when the big parties at Westminster were up to their necks in the row about MPs' expenses. Back then, 13 UKIP MEPs were elected, but only nine of them were left by 2014.
After a set of local election results the other day that some thought were simply not good enough for a party hoping to win a general election in less than a year, Labour will be hoping to jab UKIP in the eye and snatch the headlines themselves by winning the European elections.
Last time around, in 2009, Labour were in a dire position. The only region in which they came first was the north east of England. They were a poor second to the SNP in Scotland and a narrow second to the Conservatives in Wales. They limped home in third place overall, with 13 MEPs, behind the Conservatives and UKIP - a party with no MPs.
This time, they have to do much better. Beating the Tories is essential, beating UKIP would be very helpful. But they have to do so against a trend since 1999 which has seen a significant and increasing share of the European Parliament election vote going to parties other than the big three at Westminster as well as the SNP in Scotland and Plaid Cymru in Wales: 19% in 1999, 33% in 2004 and 40% in 2009.
In 2009, David Cameron said he was "delighted" with his party's performance and said the vote proved the Tories were making progress across the country. Five years ago, 27 MEPs were elected on a Conservative ticket, more than double the number from any other British party.
But, four years into David Cameron's leadership at the time, and with Labour in no end of trouble, the Conservative share of the vote increased by just one point - to 27.7% from 26.7% in 2004.
The figures are a reminder that because a wide range of parties attract significant support at European Parliament elections, a party can top the poll with a relatively small share of the total vote. Tory activists have been talking down their chances in these elections for some time, to such an extent that it is almost "priced in" at Westminster that they will finish third in these elections, behind UKIP and Labour.
Nonetheless, if they do, it will be the first time in their history that they have come third in a national election. Despite the expectation from many that this will happen, psychologically it would still be wounding less than a year from a general election they hope to win.
The Liberal Democrats
If UKIP have gone in for barely any expectation management at all, it is the opposite with the Liberal Democrats. Back in 2009, 12 Lib Dem MEPs were elected, even though the party's share of the vote dropped by a point compared to 2004.
Now with less than optimistic words like 'oblivion' 'annihilation' and 'wipe-out' so often appearing in sentences which also include the words Liberal and Democrat, expectations are pretty limited. With some suggesting the party will be left without any MEPs at all, a result which means any yellow rosettes in Brussels will attract the almost upbeat label "it could have been worse".
On the face of it, Nick Clegg's strategy for these elections seemed odd. As the leader of a very unpopular party, he proudly talked up an unfashionable view - that the European Union is good for us.
The political logic behind this apparent political masochism was that there is a chunk of the electorate that is instinctively pro-European and with most other big parties falling over themselves to sound sceptical about Brussels, the Lib Dems would have a unique selling point.
The Green Party of England and Wales
The Green Party likes the European Parliament elections. Rewind back to 1989 and it was the Greens doing the equivalent of UKIP now: grabbing the headlines and unnerving the big parties. That year they came from nowhere to take third place in the popular vote with 14.9%. But the first-past-the post electoral system for elections to Brussels and Strasbourg in place at the time meant they didn't win a single seat. Now, with proportional representation, they have a better chance.
They had two seats from 2009 to 2014. The leader of the Greens, the Australian-born Natalie Bennett, has predicted they could treble their representation and win six seats.
British National Party
One of the main headlines after the 2009 European Parliament elections was the success of the BNP. The party won almost a million votes and party leader Nick Griffin won a seat in the north west of England. Colleague Andrew Brons won in Yorkshire and the Humber.
But things have been pretty rough for the party ever since: Mr Brons left and set up his own party, its opinion poll ratings have tumbled and the vast majority of its councillors have since been voted out. The growing prominence of UKIP - a party of "plastic patriots" according to Mr Griffin - clearly hasn't helped either.
Scottish National Party and Plaid Cymru
The SNP had two MEPs between 2009 and 2014 while Plaid Cymru had one. With the independence referendum in Scotland now just a few months away, the SNP will want to demonstrate the momentum it points to in some polls on independence translates into real votes in real ballot boxes for the European Parliament elections.
Plaid finished third in Wales in 2009, with 18.5% of the vote. This was an improvement in terms of share of the vote compared to 2004.
There are plenty of them. Are you ready? Here goes: Scottish Green Party, English Democrats, An Independence from Europe, No2EU - Yes to Democracy, Christian Peoples Alliance, Socialist Party of Great Britain, Britain First, Peace Party, Animal Welfare, Communities United, 4 Freedoms, National Health Action Party, National Liberal Party, Socialist Equality Party, We Demand a Referendum Now Party, Europeans Party, Harmony Party, Socialist Labour Party, Liberty GB Party, Pirate Party and Yorkshire First.
And... the rules
The UK sends 73 MEPs to the European Parliament. The UK is divided into 12 huge constituencies. Each has between three and ten MEPs and each MEP represents everyone living in their region.
Each voter picks the one party they want to support. Seats are then allocated using proportional representation in each region, with parties ranking their own candidates into a pecking order to decide which ones get a seat in Brussels and Strasbourg.