Migration isn't a one-off event - it's a global and complex phenomenon which changes from year-to-year. The Born Abroad maps and figures aim to be both a snapshot of what was happening in 2001 while also providing a sense of the recent history of migration. But what are some of the key issues that have affected migration in and out of the UK since then?
It's virtually impossible to accurately say how many people come to live in the UK on a permanent or semi-permanent basis on a yearly basis because of the various ways people are allowed to come in and the procedures available to them to apply to settle. In simple terms, someone arriving one year on a temporary work permit, may be later granted a right to permanently stay - the difficulty is in avoiding counting that person twice.
In general terms, the number of people coming for at least one year, in relation to the number of people who were leaving, has been going up since 1987. In 1999-2000 100,000 more people came for a year than left, before reaching a high of 171,000 in 2001. That figure has dropped back since.
Another key measure is the figure for people giving "grants of settlement" - a right to stay permanently in the UK. This figure has been tracking the general trend and came to about 144,000 in 2004.
It excludes EU nationals who generally have an automatic right to stay under the free market rules that allow British citizens to do the same on the continent.
A key part of the picture is university students. Major institutions - and smaller ones too - regard foreign students as a key resource because they pay higher fees than the institutions get for British students. About 300,000 a year come at present and many universities are establishing formal links with other counties.
Students account for some of the greatest diversity to be seen in urban areas: our maps show some that some of the largest clusters of people born abroad are in neighbourhoods with university halls of residence.
The rise in the number of asylum seekers was the trigger for the ongoing controversy about migration - but in reality their numbers have fluctuated wildly over the past 10 years but generally in line with international trends.
The high point for asylum applications (excluding dependents) was 84,000 in 2002, since when it has fallen. The BBC News website archive includes a large number of stories detailing the changes over the years.
At the census, the population of the UK was 58,789,194. The latest estimate, published a week before Born Abroad was published, is 59.8m. The figures suggest that the UK population will very soon top 60m, in line with predictions. Migration is an important part of this population growth; as our Born Abroad figures show, those coming from abroad accounted for half of population growth between 1991 and 2001.
The Office for National Statistics estimates that net migration (ie those who coming from abroad minus those who are leaving) accounted for two-thirds of population growth in the year to summer 2004.
Migration is not a one-way street. As the world has become more mobile thanks to globalisation and cheaper travel, more people take the opportunity to move. While the UK has been experiencing greater immigration, it has also experienced greater emigration. This is not just people born in Britain leaving.
The figures in this project show steep declines in some of the key historic migrant groups to Britain, principally the Irish and people born in the Caribbean as many take the chance to retire back to their home countries. No accurate figures exist for the number of Britons who live abroad, how long they stay and how many return.
NEW EUROPEAN UNION STATES
One of the new trends in migration has been arrival of workers from the European Union's new eastern states. Estimates suggest the UK is the favourite destination for workers from these states, largely because the British government has introduced liberal employment rules for new EU workers to plug what it says are labour shortages in a strong economy.
There are two conflicting figures in circulation, but both suggest the UK is the favourite destination.
Whitehall thinks about 130,000 new EU workers have come to Britain since enlargement in May 2004. A German study using different methodology suggests the figure for "new workers" - those who didn't already have ties to the UK - is nearer 50,000.
By definition, it's impossible to count something you cannot see - but do we know anything about illegal immigration?
It's unlikely that many if any unauthorised migrants - thought to be mostly people who overstay their visas and continue to work in the black economy - filled in the 2001 Census form so the next best thing is guessing. The Census authorities did a degree of this themselves by trying to over-count the various groups they thought might not take part - the largest of these being single British men.
Beyond the census, a row during the general election on illegal migration figures showed how difficult it is to estimate, with the top academics in the country saying their best guesses are based on inspiration more than anything else.
The official guess put forward by the Home Office is that there are between 310,000 and 570,000 people living in Britain who aren't authorised to do so, although we have no means of working out if that's accurate.
The British work permit and visa system has been notoriously complicated for years - something which does not help people understand how migration works in practice.
Now, the Home Office is in the middle of scrapping it and over five years replacing it with what ministers pledge will be a more transparent system of tiered-entitlement based on skills.
In short, those with the least skills will have little right to settle, but will still be able to work if there is demand for their labour. Those with the most skills to offer will have greater chance to stay. How this will work in practice remains to be seen.