At 9 p.m. EST, Donald Trump, Marco Rubio, Ted Cruz, and John Kasich took the stage for the 11th debate of the cycle.
The Fox News-hosted event comes a month after Trump boycotted the previous Fox debate after complaining that moderator Megyn Kelly would be unfair to him.
Trump has fiercely attacked Kelly since the first Fox-hosted debate last summer, during which she famously confronted him over disparaging comments he's made about women.
In a Thursday-morning interview on "Good Morning America," Trump said he was "absolutely ready" to face Kelly again.
And in an interview with Business Insider earlier this week, another Fox moderator, Bret Baier, said his team believed that the whole brouhaha was "water under the bridge," and it would not be a debate topic unless other candidates brought it up.
Sens. Cruz and Rubio, who have both implicitly called for the other to drop out in order to help form a coalition against Trump, are also likely to aggressively criticize the reality-television star. There is even increased speculation about about an 11th-hour attempt to stop Trump at the Republican convention this summer.
Read our debate live-blog below:
9:12 p.m. - Kasich refused to answer a question about his path to the nomination.
"This is so much about process, it's boring to me," Kasich said, before noting that head-to-head polls show he could theoretically beat Clinton in a general election.
9:06 p.m. - Trump said that Rubio's potential insinuation that Trump's penis is small was false.
"And he referred to my hands, 'If they're small something else must be small.' I guarantee you, there's no problem, I guarantee you."
9:05 p.m. EST - Wallace asked Rubio why he shifted tactics, attacking Trump with forceful rhetorical jabs after dodging personal attacks for most of the campaign.
"This campaign for the last year, Donald Trump has mocked everyone with personal attacks," Rubio said.
"If anyone deserves to be attacked that way, it's Donald Trump."
9:02 p.m. EST - Co-moderator Chris Wallace began the debate by asking Trump about former Massachusetts Gov. Mitt Romney's recent criticisms of the former reality-television star.
"He was a failed candidate, he should've beaten Obama very easily," Trump said of Romney. "He failed miserably."
"Obviously he wants to be relevant, he wants to be back in the game."