Former National Security Adviser Michael Flynn's abrupt resignation made waves on Monday evening, as pressure mounted amid controversy over his communications with a Russian ambassador.
Nevertheless, as the principal adviser on national security issues, the opening in President Donald Trump's administration is a crucial one that the administration is most likely to fill quickly.
Here are five possible candidates that may become the next national security adviser to Trump:
Vice Adm. Robert Harward, a former deputy commander of US Central Command, is the front-runner to replace Flynn, according to Washington Post reporter Robert Costa. The New York Times also reports that Harward is the leading candidate to take over.
The position is appointed by the president, and does not require a lengthy confirmation hearing from the Senate.