More than 100 million people around the world are migrant workers seeking better opportunities in a foreign country, says the International Labour Organisation. Last year more than half a trillion dollars was sent home by migrant workers, and the amount sent to developing countries has more than quadrupled in the past decade.

Among the biggest destinations for migrant workers is the Gulf region, whose fast-growing, resource-rich countries have attracted millions of labourers from South Asia, East Asia - and, increasingly, Africa.

Workers in the six countries that make up the Gulf Cooperation Council (GCC) sent home about $61bn in 2011, the last year for which full data is available. Labourers in Saudi Arabia remit more money than those in any other country except the United States. In 2011, Gulf workers sent home almost as much money as foreign workers in Europe's three biggest economies combined: Germany, France and the UK.

India is the biggest beneficiary, receiving about $30bn in remittances from the Gulf states in 2012. "Remittance flows from the [Gulf states] provide a lifeline" to developing countries, said Dilip Ratha, the manager of the World Bank's Migration and Remittances Unit.

Although the Gulf's economic growth has provided millions of jobs to people from developing countries, states in the region and in the Middle East more broadly have been criticised for failing to protect labourers from poor working conditions and abuse. In April 2013, the United Nations' International Labour Organisation reported that an estimated 600,000 migrant workers are "tricked and trapped into forced labour across the Middle East".

Rights groups are also strongly critical of the kafala system, used across the Gulf, which binds workers to a single employer and limits workers' ability to change jobs. This system facilitates forced labour, they say, adding that domestic workers - who are not covered under Gulf countries' labour laws - are particularly vulnerable.

Diplomatic spats often flare up between sending countries whose citizens work overseas and receiving countries that hire them, especially those in the Gulf region.

Sending countries frequently complain that their workers - especially domestic labourers - are not treated fairly or paid enough at their jobs overseas. Although sending countries sometimes ban their citizens from working in certain Gulf countries, the bans are difficult to enforce. "It's much easier to regulate immigration than it is to regulate emigration," said Martin Ruhs, an economist at Oxford University who specialises in labour migration.

Ruhs told Al Jazeera that when a sending country threatens to withdraw its labour supply, "there's ... often a poorer country that is under even more pressure to maintain the flow of workers and who may be willing to step in". Accordingly, sending countries often lack the bargaining power to demand better conditions.

Some of these disputes have been heated, such as that between Saudi Arabia and Sri Lanka. But other labour bans, such as that on Ethiopian domestic workers in the United Arab Emirates, have been mutually agreed upon by the two countries.

Read more about some of these labour disputes below:

With small native populations but fast-growing economies, Gulf governments have long imported labourers from abroad to help build their countries.

But some worry that heavy reliance on foreign workers - who outnumber the native population in every Gulf country but Saudi Arabia and Oman - is causing unemployment among Gulf countries' citizens and having negative effects on society.

Private companies in the region are especially reliant on migrant workers, who are often willing to work long hours for low wages. Accordingly, most Gulf citizens in the labour force work in public - sector jobs.

A report by the secretariat of the Gulf Cooperation Council in 2009 said, "More and more GCC citizens are graduating from universities and institutes and cannot find jobs. It is time for the GCC countries to intensify their efforts to find jobs for their people ... otherwise it will be too late."

Here's what some of the Gulf countries have been doing about the low numbers of citizens in the private-sector workforce: