Why the European Union matters in this election
The UK's relationship with Europe has always been rocky. But the next five years will decide whether Britain has any future in the EU at all.
What's at stake?In five words: British membership of the EU.
It's less easy to summarise what this means economically - two recent estimates have suggested more than four million UK jobs are associated with exports to EU countries but the reports say nothing about dependency on membership of the EU.
Britain, as part of the EU, is also signed up to freedom of movement between member states, certain rules designed to protect consumers and employees, the free market competition rules, and EU-wide justice measures such as the European Arrest Warrant.
Leaving would give Britain more powers to limit migrant worker numbers, but it is unclear what the status of EU workers already living in the UK would be.
Arguments for renegotiation or reform from within the EU tend to focus on measures to curb freedom of movement - but European Commission President Jean-Claude Juncker and German Chancellor Angela Merkel are among those who have warned that that would threaten a core EU principle.
142,000 more EU migrants entered the UK than left in the year to June 2014 (ONS)
The UK contributed £8.6bn more to the EU in 2013 than it took out (HM Treasury)
The UK spent nearly 8,000 euros in social security per inhabitant in 2010 - this compares with nearly 9,000 euros spent by Germany and France, over 10,000 in Denmark and the Netherlands - or less than 2,000 in Bulgaria and Romania (Eurostat)
Average time of extradition proceedings before the European Arrest Warrant was one year, but that has been cut to an average of 48 days (European Commission)
Few people predicted how enormous the impact of the Scottish independence referendum would be on the entire British political landscape - and the impact of a referendum on the UK's membership of the EU has the potential to be even more dramatic.
If it does take place, it is likely to expose deep divisions within the Conservative Party. And if it doesn't, how will Labour escape accusations that it is behaving anti-democratically?
Could UKIP survive a vote to stay in? Would a vote to leave trigger demands for a second Scottish independence referendum?
That's just the domestic picture. Pro-EU groups tend to argue that the UK's influence would diminish outside the bloc - or that renegotiation could place unsustainable strains on its membership.
Eurosceptic thinkers have judged that a Brexit would not damage the Britain's international standing, since it would remain a key part of Nato and the UN Security Council and a nuclear power, with a powerful global voice in its own right.
- UKIP won big at 2011 local elections and then again at the 2014 European elections, becoming the UK's largest party in Europe - they also gained two MPs, defectors from the Conservative Party
- In 2013 Conservatives promised to offer EU referendum if re-elected, after pressure from UKIP and inside the party (a handful of Labour MPs also favour a referendum)
- Government has opted in to 35 EU justice and home affairs measures - including the European Arrest Warrant - after opting out of more than 100 in 2014
- David Cameron forced vote of EU states on the selection of Jean-Claude Juncker as European Commission chief and lost by 26 to two.
"There is an overwhelming sense that globalised elites have broken free from national loyalties... Until mainstream politicians learn how to grapple with that feeling, they will not be able to tackle the anti-politics mood that is sweeping Europe" - Mark Leonard, director of the European Council on Foreign Relations
"In a Europe struggling with sluggish growth, weak governments and rising populists, an Anglo-German bargain for sweeping EU reform is no longer only desirable. It is absolutely necessary" - Mats Persson, director of Open Europe
Cameron and Miliband face TV grilling