What does US immigration overhaul mean?
US President Barack Obama has announced the biggest overhaul of the country's immigration system in years.
The centrepiece of the reform is a plan to offer millions of illegal immigrants the chance to get a three-year work permit.
The president has long described the present system as "broken", and said in a televised speech that he was forced to take executive action after Congress failed to pass an immigration bill.
How is the system 'broken'?
There are currently an estimated 11 million undocumented immigrants in the US, many of whom work in cash-in-hand jobs and therefore do not pay taxes.
Hundreds of thousands of people try to cross the border from Mexico each year, many fleeing poverty in Central America or seeking to join relatives.
Tens of thousands of unaccompanied children make the journey, and cause a particular problem for the US. Many are held in crowded detention centres while US officials struggle to hear all of their cases.
Mr Obama has called the issue a "humanitarian crisis".
What do we know about immigrants in the US?
The number of undocumented immigrants in the US has risen from an estimated 3.5 million in 1990 to just over 11 million, according to research by the Pew Research Center.
More than half of immigrants arriving illegally in the US are from Mexico, with the rest arriving largely from Latin America and Asia, the Department of Homeland says.
The US deported 369,000 people in 2013 alone, with 400,000 deported the year prior.
But Mr Obama has argued that it would be impossible to deport all of the illegal immigrants.
What has Mr Obama done?
He has used executive action - in other words, his power as president - to dodge the political deadlock in Washington and push through reforms without requiring a vote in Congress.
Although the package includes promises to facilitate legal immigration, toughen border security and refocus deportation efforts on to criminals, the centrepiece is the offer of work permits for illegal immigrants living in the US.
To apply, people must have been living in the US for five years, and have a child who was US born or holds citizenship.
A quirk of the US system is that any child born in the US automatically becomes a citizen, resulting in parents with no legal status bringing up children with full citizenship.
How many will be affected?
Some 3.7 million people are thought to be eligible for the scheme allowing parents work permits, according to the Migration Policy Institute.
Another part of Mr Obama's plan will expand the Deferred Action for Childhood Arrivals (Daca), a programme that started in 2012.
Some 1.2 million people are already eligible, but Mr Obama has broadened the criteria, potentially expanding the scheme by 300,000.
Why is this such a big political issue?
Republicans have been highly critical of Mr Obama's use of executive orders to force through reforms that Congress will not vote through.
They say he is acting beyond his powers, but it is not clear whether they will ask judges to review the order.
An immigration bill passed the Democrat-led Senate in 2013, but Republican House leader John Boehner refused to even debate the proposal.
The Democrats have since lost control of the Senate, making the bill's passage even more difficult.
Mr Obama says the order is within his power as president, and blamed the Republicans for forcing him to use his executive power.