Why immigration matters in this election

Immigration was one of the top subjects of public debate in the 2015 general election - but amid all the arguments and controversy, what do we know for sure about what is going on?

What's at stake?

Immigration has undeniably played an important role in shaping many parts of modern Britain. London, for instance, is now one of the most diverse cities on the planet, a place where scores of languages can be heard on the streets.

It began in the decades after the World War Two, with largely Commonwealth workers filling labour shortages. The NHS is the most obvious sector of society that is reliant on immigration: around a third of its workforce was born abroad. Today's immigrants are, generally speaking, young and mobile workers or students, and increasing numbers of them come from other parts of the European Union.

But the big unanswered question about migration is whether its forces have a wider positive or negative impact on the economy and society.

Do foreign workers price home-grown labour out of the market place? Does their arrival mean that society, government and business avoids difficult decisions about how to deal with unemployment, benefits systems and low-skills?

What role does fast-changing immigration play in key decision about planning for health, education, housing and other services?

What are the numbers?

On the government's preferred measure, 298,000 more people came to live in the UK than left in the year ending September 2014.

The figures show that the most likely reason for someone to come to the UK is to work or study. While asylum seekers arrived in large numbers a decade or so ago, they form a small part of the overall picture today. This is almost 200,000 over the target the prime minister set when he came to power.

Overall, there were almost one million people either moving in or out of the UK over the course of the year.

What won't the politicians be saying?

The figures may be affected one way or another by specific policies but they are also part of a historic shift in how the industrialised open economies of the world operate. Wherever there are free market places, there is movement of people. The UK is not alone in experiencing these changes - and indeed helped to create them by backing the European free market 25 years ago.

Nobody knows for sure how many migrants there are because the UK still hasn't got a single and reliable method to count them in and out. In turn, that means it's very difficult to find hard data to support or dismiss claims of the effects they have on society, allegations of benefit tourism and, hardest of all, estimate the number of immigrants living here illegally.

Most politicians in England are unlikely to talk about why most politicians in Scotland want more immigration.

What has happened since 2010?

• Government legislated to restrict certain categories of immigrants

• Numbers began to drop but then started to rise again

• Home secretary has brought the workings of the system under direct ministerial control

What do the experts say?

"The UK is definitely one of the most attractive destinations in Europe. It is quite a flexible labour market and has far more jobs at the low end of the economy which makes it easy for migrants to find and get a job compared with a country like Germany" - Madeleine Sumption, Director of Migration Observatory, University of Oxford

"We will, in the next 25 years, have to build the equivalent of ten cities the size of Birmingham. This would place enormous stress on our already creaking infrastructure and on our environment and it would also change the nature of British society for ever. We must not sleep walk into one of the most significant changes in a thousand years of our island's history" - Sir Andrew Green, Migrationwatch UK