By David Loyn
Developing world correspondent, BBC News
For a look at what the new world of migrants really means, take a walk around the old town in Doha any Friday afternoon.
Development in Doha is built on the foundations of migrant labour
Near the Wind Tower, an original desert house designed to draw cooling air through its mud walls, thousands of men gather to meet, sit, walk and talk.
They wear their best clothes, filling every available space on the marble avenues and squares that are the preserve of shoppers the rest of the week.
The men are migrant workers on their only day off. They come mainly from India, Bangladesh and Nepal, and from places further east in Asia.
Doha, the capital of the Gulf state of Qatar, is one of the handful of world cities where more than half of the population is made up of migrant workers.
The vast majority are cleaners and builders, who travel in and out of the desert kingdom on flights from South Asia like worker bees, already wearing the company uniform.
The economics of this trade benefit both sides.
The oil-rich Gulf states have a workforce willing to earn less than people from developed countries, while the workers have an income that is far more than they could earn in their home villages.
One-quarter of the migrant worker population come from just two countries: India and China, the other emerging global economic superpower
To see the difference this can make, go to Kerala, the Indian state that "exports" more workers than any other.
It benefits by more than $5 billion (£2.6bn) from them annually, a huge stimulus to the local economy.
And the workers are not all at the bottom end of the wage scale.
Indian accountants and managers are increasingly running things in the Gulf and elsewhere in the world.
India is also one of the world's largest hosts of migrant workers, with millions of workers, particularly from its neighbours, coming for casual building and farm labouring jobs.
A quarter of the migrant worker population come from just two countries: India and China, the other emerging global economic superpower.
The Chinese are working in industry at all levels around the world now, particularly in Africa.
In the land-locked mountain kingdom of Lesotho, I encountered Chinese foremen in a textile factory talking conversationally to local workers in the local language.
The factory was at the glamorous end of the fair trade spectrum, making branded t-shirts for a new line promoted by the rock star Bono.
These are the kinds of examples that Brunson McKinley, director-general of the International Organisation for Migration, is talking about when he says that migrant workers are now "an essential, inevitable and potentially beneficial component of the economic and social life of every country and region".
Remittances from migrant workers to the poorer countries of the world continue to be one of the major drivers of international development.
The amount returned to developing countries by expatriate workers, at more than $160 billion (£82bn), is far higher than the total aid budget spent by the developed world on developing countries, which runs at around $100 billion (£51bn) a year.
A changing world
But as the IOM also point outs on this year's International Migrants Day, migrants are often the forgotten victims when disaster strikes.
While Britain and other European countries were sending warships to bring their citizens out of Beirut when Israel attacked Lebanon this summer, 11,000 migrant workers, mainly domestic servants, were left stranded and needed separate funding for an emergency evacuation.
Hundreds of thousands are fleeing from the worsening catastrophe of Iraq, putting extreme pressure on Iraq's neighbours.
Thousands of African migrants make a dangerous journey to Europe
Already Jordan has imposed severe new restrictions, turning back men of military age, for fear that the conflict could spread across the border.
And there are other problems too.
The deaths of at least 21 Chinese cockle pickers, caught by the tide at Morecambe Bay in the UK in February 2004, shed a tragic light on the unregulated world of casual labourers.
New laws brought in since then have attempted to control gangmasters in Britain who were exploiting the workers.
Many migrant workers live outside the law, and cannot benefit from health and safety rules designed to protect employees
But many migrant workers live outside the law, perhaps up to 20% of the immigrant population worldwide and almost one-third of non-citizens in the US.
They cannot benefit from health and safety rules designed to protect employees.
There are believed to be as many as eight million unregulated immigrants in Europe, and becoming an "economic migrant" can literally be a life and death decision.
The now-common sight of bodies of Africans washed up on European beaches after trying to make a hazardous crossing shows the lengths to which people will go to try and move to a new life.
But those who make it are transforming the feel and look of cities in the newly mobile globalised world.
A country like Ireland shows the difference that this kind of mobility can make. Ireland used to be an "exporter" of migrant workers, who left its shores for a better life.
It is now one of the most popular countries for migrant workers to settle.
A traditional bar in Dublin is now as likely to be managed by someone from Vietnam, or Poland as a native-born Irish citizen.
To coincide with International Migrants' Day, BBC News is launching World on the Move, a special series on the growing phenomenon of migration. You can see more reports on this website in the coming days.