It's a question that has dogged her campaign for over a year - but opinions are divided over whether the allegations made against her constitute a crime or are just the latest partisan sideshow.

Perhaps the best way to look at the implications of her case is by considering the context of another high-profile legal drama involving classified documents that was recently resolved - that of former CIA director David Petraeus.

Petraeus pleaded guilty last year to a misdemeanour offence, mishandling classified information, after being accused of handing notebooks with classified information to his biographer-turned mistress, Paula Broadwell.
He was fined and placed on probation - a resolution that some have called a slap on the wrist.

In other cases people who have revealed classified information were sent to prison. In 2009 Stephen Kim, a former government contractor, was sentenced to 13 months for giving classified material to a reporter.
But Petraeus is a high-profile figure. He has been credited with implementing the US military "surge" in Iraq that helped bring stability to the nation - at least temporarily. When President Barack Obama tapped him to head the CIA in 2011, his name was kicked around by some Republicans as a possible presidential candidate in 2012.

His celebrated background has led critics to suggest that there is a double-standard in cases involving the mishandling of classified information - one making it less likely that Mrs Clinton will face repercussions for her actions.

Mrs Clinton used a private server at her house while she was secretary of state, and some of her emails appeared to have contained classified information, though it's unclear whether that information was classified at the time it was sent. Revealing classified information is a crime.
Yet the offence is treated in different ways, depending upon the circumstances. They're also tough cases to prosecute.

Government lawyers have to prove an individual knew what he was doing when he revealed classified information, for example, or that she was unusually sloppy when handling the material