Robert Mueller And The Probe Heard 'Round The World

He's leading one of the probes into Trump's ties to Russia.
Who Is Robert Mueller?
Robert Mueller served as FBI director under presidents George W. Bush and Barack Obama before going back into private practice. In May, he was chosen as special counsel to lead the Justice Department's Russia investigation. He's since filled his team with a number of high-profile attorneys from both the public and private sector.

In August, Reuters reported that Greg Andres, a former DOJ official who specialized in fraud and foreign bribery cases, was the 16th member of Mueller's unit. Legal experts told Reuters that the fact Mueller is still staffing up suggests his investigation could just be getting started, and may go on "well into 2018."
What Is He Investigating?
Mueller has officially been tasked with investigating "any links and/or coordination between Russian government and individuals associated with the campaign of President Donald Trump, and any matters that arose or may arise directly from the investigation."

Although the investigation specifically revolves around Russian-led election interference and communications between Trump officials and the Russian government during the campaign, it has evolved with recent developments. Mueller's team has expanded the probe to examine whether White House officials obstructed justice by attempting to improperly influence investigations into Flynn's conduct or whether Trump did so by firing Comey. Investigators have also begun looking into Trump's finances.
How Did Trump Respond?
Trump has falsely claimed that Mueller's team is filled with "Hillary Clinton supporters," who have embarked on a "witch hunt" to derail his presidency. In an effort to discredit them, the president has reportedly dispatched lawyers and aides to dig up dirt on the investigators, which could be used as grounds to fire Mueller or oust certain members of his staff. Trump also told The New York Times that he believes Mueller would be overstepping if he looks at Trump family finances that aren't directly related to questions about Russia's involvement in the election.
What Was The Fallout?
Many have seen Trump's eagerness to criticize and combat Mueller as evidence that he is intent on burying the Russia investigation by any means necessary. While legal experts are split on whether Trump could technically fire Mueller -- and what it might take to justify such an action -- many agree that it would invite a constitutional crisis and could even potentially serve as grounds for the president's impeachment.

But the investigation's significance is clearly more than symbolic. In July, FBI agents raided Manafort's home in Virginia, and seized “documents and other materials," the Washington Post reported in August. Manafort had retroactively registered as a foreign agent a month before the raid, following reports that his firm had been paid more than $17 million from 2012 to 2014 for lobbying work on behalf of a pro-Russian political party in Ukraine.