Clinton has participated in dozens and dozens of debates over her two presidential bids, and Trump debated a handful of times in his march to the Republican nomination.
So, what do we know about these two as debaters on the verge of the biggest moment of their political lives? Let's break it down.
HILLARY CLINTON
As with most elements of her candidacy, Clinton very rarely dazzles but almost never disappoints as a debater.
She is always briefed on policy to the hilt and able to retrieve pertinent information when the moment requires it. If you close your eyes and think of the best student in your high school class - relentlessly prepared, always attentive - that's Clinton in a debate.
Her preparedness does occasionally work against her - as does her tendency towards lawyerly rather than political answers.
Clinton can come across as overly rehearsed and sometimes get way in the weeds on policy for the average undecided voter. She also sometimes comes across as overly cautious and legalistic in the way in which she answers questions - a carefulness that her allies insist the moment demands and that her detractors view as Exhibit A in why they don't trust her.
Clinton's biggest - and, if we are being honest, really only - major debate stumble over the past eight years came in the northern autumn of 2007 during the Democratic primary season when she tried to delicately hedge her answer to a question on whether illegal immigrants should have driver's licenses.
Clinton seemed to say she supported the policy of Eliot Spitzer, who was the Democratic Governor of New York at the time, to grant those licenses, then she rapidly reversed course.
Her opponents - led by senators Chris Dodd and Barack Obama - savaged Clinton over the wishy-washiness. That moment began a long erosion in her support that culminated with Obama's victory in the Iowa caucuses.
DONALD TRUMP
Because Trump has never run for public office before, we have a relatively small sample to draw from when analysing his performance.
That said, during the primary debates, Trump appeared to have only two settings: Attack Mode and Disappearing Mode.
When in Attack Mode, Trump would unleash invective at whoever was in his way - regardless of whether it made political sense. (I distinctly remember a debate where Trump took a shot at Senator Rand Paul, who was barely clinging to the stage and relevancy in the race.)
Trump also goes into Attack Mode whenever he believes he has been challenged or disrespected. His need to "guarantee" that he has "no problems down there" in response to Senator Marco Rubio's attacks referencing his hand size was vintage Trump in all-out Attack Mode.
Then there is Disappearing Mode Trump. This is the side of Trump as debater that doesn't get talked about as much but was often on display in the primary debates.
Trump would have flashes of Attack Mode, but then, often as the debate wore on, he would seem to fade from the stage as other candidates debated policy specifics. Trump's disappearing act was all the more remarkable because he was almost always smack dab in the centre of the stage thanks to his strong position in the polls.
What Trump does well in debates is exactly what he does well as a candidate: He keep his opponents guessing. His unpredictability - what will he say? when will he say it? - stands at the heart of his appeal and is incredibly difficult to game-plan for. There is, after all, only one Donald Trump.
What's the one thing that each candidate needs to do?
Clinton needs to expose Trump as a policy lightweight and a dangerous potential president while avoiding coming across as overly prepared or sanctimonious.
Trump needs to demonstrate some command of issues and resist being goaded into the sort of personal attacks that will almost assuredly backfire against Clinton.
