US President Barack Obama has announced he will defer deportation and provide the chance of better jobs for about five million undocumented immigrants, ending months of build-up and initiating a showdown with congressional Republicans.
Without a vote from Congress, Mr Obama set in motion a controversial program that will evaluate applicants from next year, though it offers no path to citizenship.
Here's what you should know about the immigration overhaul.
Who gets relief?
- 4.1 million undocumented immigrants who have been in the United States for more than five years and have children who are US citizens or legal permanent residents. They will get relief from deportation for three years and be able to obtain work permits;
- 270,000 undocumented immigrants who went to the US illegally as children will be eligible for the Deferred Action for Childhood Arrivals (DACA) programme;
- 400,000 highly-skilled workers will be eligible for visas;
- Some other smaller categories for relief will bring the number affected to about 5 million.
Who gets left out?
- Undocumented parents of DACA recipients will not be eligible for legal status;
- Undocumented agricultural workers will not be addressed.
What Republicans say?
Republicans have warned that unilateral action from Mr Obama ruin chances for both sides to work together in the new Republican-controlled Congress. Many Republicans oppose both the decision by the President to act alone as well as the way he is tackling the issue. Many want border security to be the top priority and also demand that illegal immigrants be deported rather than given work permits.
Senator Mitch McConnell, the incoming Senate majority leader, issued a final warning. "He needs to understand something. If President Obama acts in defiance of the people and imposes his will on the country, Congress will act. We're considering a variety of options. But make no mistake. When the newly elected representatives of the people take their seats, they will act," he said.
What analysts say?
Although Mr Obama is not breaking new ground by using executive powers to carve out a quasi-legal status for certain categories of unauthorised immigrants, his decision will affect as many as 5 million immigrants, far more than the actions of Republican presidents Dwight D. Eisenhower, Ronald Reagan and George H.W. Bush.
Mr Obama's action is also a far more extensive reshaping of the nation's immigration system.
"The magnitude and the formality of it is arguably unprecedented," said law professor Peter J. Spiro from Temple University. "It's fair to say that we have never seen anything quite like this before in terms of the scale."
The breadth of Mr Obama's decision is already raising serious legal and constitutional questions, fueling Republican charges of imperial overreach and worries among some Democrats of future fallout.
Some legal analysts said Congress could struggle to win a lawsuit to overturn the action since presidents have historically had broad authority to act on immigration.
"When an issue is mostly political, the judicial branch generally is not going to want to step in the middle of a dispute between the executive branch and the legislature," said Mr Ted Ruthizer, an immigration attorney at Kramer Levin.
What could happen next?
How the Republicans choose to proceed in their opposition to Mr Obama's directive will shape the final two years of his tenure and could help set the tone of the 2016 presidential campaign.
Several Republicans have said they will use a coming spending Bill and the threat of a government shutdown as leverage against Mr Obama, while others in the party are reaching for ways that Congress might undercut the president's actions by withholding money or threatening other priorities.
"By ignoring the will of the American people, President Obama has cemented his legacy of lawlessness and squandered what little credibility he had left," House Speaker John A. Boehner said in a statement after the speech.
Even as Republican lawyers analysed what the White House said was the legal basis of Mr Obama's actions, it remained unclear how they might undo the decisions.
The agency that will carry out most of the president's executive actions - the Citizenship and Immigration Services - is funded with application fees, and does not rely on a budget vote in Congress to keep operating.
Source: ABC News, Los Angeles Times, New York Times, Bloomberg