Donald Trump promised during the primary race that he would halt Muslim immigration to the United States in an attempt to prevent terrorist attacks on American soil.
"Donald J. Trump is calling for a total and complete shutdown of Muslims entering the United States until our country's representatives can figure out what the hell is going on," he said at a December 2015 rally.
In the general election, he changed his position, instead calling for a ban on immigration from regions with a history of terrorism. But Trump hasn't explicitly backed off his Muslim ban proposal, and it's still on his website.
WHY HE'S PROMISING IT
Trump is concerned that foreign Muslims have hostile views of the United States and that the government is unable to thoroughly vet immigrants from conflict areas like Syria.
He made the promise days after a married Muslim couple went on a shooting spree in San Bernardino, Calif., killing 14 people. The husband was born in the United States to Pakistani immigrants, and the wife was a legal immigrant born in Pakistan.
WHAT NEEDS TO HAPPEN
Trump will have broad authority to stop certain groups from immigrating to the United States. Past presidents banned Iranians during the Iran hostage crisis, as well as known communists during the Cold War.
The president gets this authority from the Immigration and Nationality Act of 1952. The law says the president can "suspend the entry of all aliens or any class of aliens."
WHAT'S STANDING IN HIS WAY
Politics likely would block a full Muslim ban. Leaders in both the Democratic and Republican parties criticized the proposal when Trump first floated it. Congress could take away his authority under the Immigration and Nationality Act.
"What was proposed yesterday is not what this party stands for, and more importantly, it's not what this country stands for," said House Speaker Paul Ryan, R-Wis., in a Dec. 8, 2015, press conference.
Experts say it would be difficult to implement a Muslim ban because of the inability to definitively verify someone's religion.
If Trump were to implement a ban on all Muslims, the United States would be the only country to have a religion test at its border.