These are some of the most memorable moments from the showdown in New Hampshire:
"Yes, I apologize."
In response to his campaign's breach of Clinton campaign voter data, Sanders apologized to Clinton. "Not only do I apologize to Secretary Clinton, and I hope we can work together on an independent investigation from day one, I want to apologize to my supporters. This is not the type of campaign that we run."
Sanders, however, did not back down from criticizing the Democratic National Committee's suspension of his campaign's access to voter data. "That is an egregious act," he said.
"We should move on because I don't think the American people are all that interested in this."
Just as Sanders believed people were "sick and tired" of hearing about Clinton's emails, Clinton herself called for an end to discussing the Sanders data breach. "I think they're more interested in what we have to say about all the big issues facing us," she added.
"Guns in and of themselves, in my opinion, will not make Americans safer. We lose 33,000 people a year already to gun violence. Arming more people to do what I think is not the appropriate response to terrorism."
Clinton voiced her opposition to calls for broader gun rights in the wake of recent attacks. She called for greater coalition building instead.
“Whoa, whoa, whoa. Let's calm down a little bit, Martin."
Sanders said that in response to O'Malley, who called out his two opponents on their gun control records. His comments were met by incredulous reactions from both Clinton and Sanders.
“Senator Sanders voted against the Brady bill. Senator Sanders voted to give immunity to gun dealers," O'Malley said. "And Senator Sanders voted against even research dollars to look into this public health issue," he said. "Secretary Clinton changes her position on this every election year, it seems, having one position in 2000 and then campaigning against President Obama and saying we don't need federal standards.”
Clinton responded by saying, “Let's tell the truth, Martin.”
“He is becoming ISIS’s best recruiter.”
Clinton condemned Donald Trump’s inflammatory statements and policy proposals on Muslims. “They are going to people showing videos of Donald Trump insulting Islam and Muslims in order to recruit more radical jihadists,” she said.
"I know Secretary Clinton was gleeful when Gaddafi was torn apart."
O'Malley criticized Clinton's tenure as secretary of state.
"The United States is not the policeman of the world."
Responding to Clinton's call for American leadership, Sanders criticized America's intervention in the Middle East. He said the U.S. "must not be involved in perpetual warfare" in the region.
Clinton apologized for taking the stage mid-question after returning late from the commercial break.
When asked if corporate America should love her, Clinton responded with this zinger.
"They ain't going to like me. And Wall Street is going to like me even less."
Sanders, unlike Clinton, said that corporate America would not like him at all.
"I have demonstrated the ability to have the backbone to take on Wall Street in ways that Secretary Clinton never, ever has."
O'Malley sharply criticized Clinton for her alleged "cozy relationship" with Wall Street. His critique was echoed by Sanders.
"I've got the scars to show from the effort back in the early '90s."
Clinton alluded to her past fight for health care in defense of the "glitches" in the Affordable Care Act.
"And in our party, unlike the Republican party, we actually believe that the more our people learn, the more they will earn."
O'Malley criticized Republicans for their education policies.
"Now this is getting to be fun."
Sanders seemed to enjoy the back and forth between Clinton and moderator David Muir.
"I think this is one of the most important challenges facing not just our next president but our country."
When asked about the state of race in America, Clinton spoke of the need to address and fix "systemic racism and injustice and inequities."
"We need to do it now as a nation. This is our time and our opportunity to do that."
O'Malley called for an end to excessive lethal force by police officers.
"We need major, major reforms of a very broken criminal justice system."
Sanders named a lack of diversity in police departments, mandatory minimums, and excessive lethal police force as some of the problems.
"We need to understand that addiction is a disease, not a criminal activity."
Sanders advocated for a massive shift in the way addiction and drug use are dealt with in America.
"When somebody is addicted and seeking help, they should not have to wait three, four months in order to get that help," he said.
"Well, there's always a retrospective to say what mistakes were made."
Clinton refused to concede whether she bears any responsibility for the chaos in Libya.
"And with respect to my own husband, I am probably still going to pick the flowers and the China for state dinners and stuff like that."
Clinton said she would turn to husband Bill "for special missions, for advice, and in particular how we're going to get the economy working again for everybody" if she became president.
"Given the fact that she's a lot smarter than me, yes, she would."
Sanders said that his wife Jane would indeed have a desk in the West Wing.
"Let me...take this moment to congratulate Hillary Clinton, who I thought not only did an outstanding job as our First Lady, but redefined what that role could be."
Sanders thanked Clinton for her time as First Lady.
"On our worst day I think we have a lot more to offer the American people than the right-wing extremists today."
Sanders took time in his closing statement to make a jab at the GOP presidential candidates. Clinton and O'Malley also denounced the Republican party in their closing statements.
"Thank you, good night, and may The Force be with you."
Clinton channeled "Star Wars" to close out the night.