Hillary Clinton was "extremely careless" using a personal email address to handle sensitive information, but she didn't do anything illegal, the FBI has found.
"Our judgment is that no reasonable prosecutor would bring such a case," said director James Comey.
The FBI says there is no "clear evidence" Ms Clinton used her private email server to intentionally send or receive classified information, so no laws were broken, and will recommend the US Justice Department not press charges.
Attorney General Loretta Lynch said last week she would accept whatever recommendation the FBI came up with.
The investigation took a year, and looked at 30,000 emails. While it found 110 emails containing classified information, it didn't believe they were sent or received intentionally, and Mr Comey said it was possible hostile foreign agents had gained access to her server.
The news will come as a relief to Ms Clinton, who is a former US Secretary of State and the presumptive Democratic nominee for President.
"As [Ms Clinton] has long said, it was a mistake to use her personal email and she would not do it again. We are glad that this matter is now resolved," said a spokesperson for campaign, Brian Fallon.
Her rival Donald Trump, the presumptive Republican nominee, wasn't so happy.
"The system is rigged. General Petraeus got in trouble for far less. Very very unfair! As usual, bad judgment," he tweeted.
"FBI director said Crooked Hillary compromised our national security. No charges. Wow! #RiggedSystem."
Mr Trump is facing legal action of his own, facing a federal lawsuit alleging he raped a 13-year-old in 1994. Mr Trump has denied the claim.