Migrant workers send as much as $600bn a year back home, according to the World Bank. Experts say the money is vital to developing economies - but the global financial crisis means the cash flow is already beginning to slow.
These vast and difficult to track money flows can represent as much as three times the total sums sent by governments as overseas aid and by businesses as foreign direct investment.
For the first time since these money flows - known as remittances - have been measured their growth is predicted to fall to more or less zero in 2009.
For some African countries, like Yemen and The Gambia, remittances amount to more than 5% of their Gross Domestic Product.
There are over 50 million migrants from Asia and Oceania worldwide. Together, the two regions receive the highest total of remittances in the world - an estimated $113bn each year.