After months of debate, restrictions to the UK labour market will finally be lifted for Romanians and Bulgarians today.
Amid reports of fully booked flights and coaches, some have predicted a surge in migrants from the two countries at a rate of 50,000 a year, with all the inevitable pressure that would place on public services in this country.
Others suggest the numbers will be much lower and the influx will boost the economy.
Here is a look at some of the key issues:
What's actually happening today?
Bulgaria and Romania joined the European Union in 2007, meaning their citizens gained the right to visa-free travel to the UK. However, temporary curbs were placed on the type of work Romanians and Bulgarians could take.
These restrictions meant employers had to apply for work permits and migrants for an accession worker card.
A number of quota schemes existed - such as the seasonal agricultural workers scheme - to allow Romanians and Bulgarians to hold short-term employment in the UK.
The curbs are being lifted because under treaty rules they cannot be extended any further.
As well as jobs, what else will they be entitled to?
Bulgarians and Romanians will also be entitled to claim the same benefits and NHS care as other EU citizens.
David Cameron has rushed through a plan to introduce a three-month wait before EU migrants can claim out-of-work benefits.
But the reforms do not affect welfare payments for claimants with jobs, meaning migrants will still have more in-salary and in-work benefits than in any other major European country, according to campaign group Migration Watch UK.
Which other EU countries imposed job restrictions?
Eight other countries imposed similar curbs, which are being lifted at the same time. They were Austria, Belgium, France, Germany, Luxembourg, Malta, Spain and The Netherlands.
What concerns have been raised?
A range of fears have been expressed, from limiting the number of jobs available for British workers to taking advantage of the benefits system, and from so-called NHS tourism to a predicted surge in crime and homelessness.
In the face of a surge in popularity for the anti-European Union (EU) UK Independence Party (Ukip), which campaigns heavily on immigration, the Government has sought to show it is not a soft touch on immigration.
However, it is attempting to balance a tougher stance on migrants with keeping Britain open to skilled migrants and international trade.
Mr Cameron says he wants to make sure people come to the UK "for the right reasons", not just to claim benefits.
How many Romanians and Bulgarians are expected to arrive?
This has been one of the most controversial aspects of the debate in the last year. The Government has refused to produce official estimates as it feels the range of variable factors will make any figures put forward too unreliable. Immigration minister Mark Harper has described such an attempt as a "fool's errand".
However, other groups have attempted to draw their own estimates. Migration Watch predicts an average of 50,000 every year for the first five years after restrictions are lifted - equal to the population of Newcastle.
Has the UK seen anything like this before?
In 2004, eight other EU countries, including Poland, joined the EU. At that time, only the UK, Ireland and Sweden opened their doors to Eastern European workers. Unofficial estimates have placed the Polish population in the UK at more than one million.
Former Labour Home Secretary Jack Straw recently admitted opening the borders to Eastern European migrants was a "spectacular mistake". The last Labour government predicted that only 13,000 would move from Poland and other eastern European countries.