Here's a quick look at what happened:
* On the Republican side, Texas Sen. Ted Cruz held on to win the Iowa caucuses against a strong challenge from businessman Donald Trump, who was the frontrunner in the polls leading into Monday night's contest.
* It was an unexpectedly tight race for second place in the GOP, with Florida Sen. Marco Rubio giving Trump a run for his money. With 99% of precincts reporting on Tuesday morning, Trump had 24.3% of the vote and Rubio had 23.1%.
* Meanwhile, former Secretary of State Hillary Clinton narrowly edged out Vermont Sen. Bernie Sanders in an extremely tight race. With 99% of precincts reporting on Tuesday morning, Clinton had 49.9% of the vote while Sanders had 49.6%.
* Clinton will get at least half of the 44 delegates awarded, according to the Associated Press. She currently has 22, and the official winner of the caucus will get the final delegate.
* Iowa state Democrats said the results are the closest in caucus history. Sanders proclaimed the close result "a virtual tie."
* Democratic contender Martin O'Malley, the former governor of Maryland, announced that he will suspend his campaign after a poor showing. And GOP contender and former Arkansas Gov. Mike Huckabee also suspended his campaign.
Here's the rundown on the Democratic side of things:
Former Secretary of State Hillary Clinton eked out the narrowest of victories in the Iowa Democratic caucuses against Vermont Sen. Bernie Sanders early Tuesday morning.
"After thorough reporting — and analysis — of results there is no uncertainty and Secretary Clinton has clearly won the most national and state delegates," Clinton's campaign said in a statement. "There is no outstanding information that could change the results."
Clinton had held a razor-thin lead deep into Tuesday morning. In at least six precincts across Iowa Monday, Democratic caucus votes ended in a tie, forcing a coin toss to decide which candidate would win a county delegate.
In her speech, Clinton said she was "breathing a big sigh of relief."
"I want you to know I will keep doing what I have done my entire life," Clinton said. "I will keep standing up for you, I will keep fighting for you, I will always work to achieve the America that I believe in, where the premise of that dream that we hold out to our children and our grandchildren never fades but inspires generations to come."
"I am a progressive who gets things done for people. I am honored to stand in the long line of American reformers," she said, then rattled off a long list of priorities: universal health care, education for all, affordable college.
"And I know we can protect our rights: women's rights, gay rights, voting rights, immigrant rights, workers' rights," she said.
While Clinton was able to eke out victory, Sanders was able to get his insurgent campaign to turn out and make it a tough night for her.
Here was the feeling at the Clinton event after her remarks:
Sanders spoke to a raucous crowd who chanted "feel the Bern!" and went wild when he said "what Iowa has begun tonight is a political revolution."
"Nine months ago, we came to this beautiful state," Sanders said. "We had no political organization, we had no money, we had no name recognition, and we were taking on the most powerful political organization in the United States of America. And tonight, while the results are still not done, it looks like we are in a virtual tie."
Sanders hit on many of his central themes, like how the economy is rigged for the wealthy.
"Our government, the government of our great country, belongs to all of us and not just a handful of billionaires," he said. "When that happens we will transform this country."
The results come eight years after Barack Obama beat Clinton in Iowa — and in the process set off a chain reaction of voters shifting from Clinton to the first-term senator. This time around, Clinton and her campaign successfully worked to correct any mistakes.
Though Clinton organized heavily in the state in 2008, this season her campaign emphasized organizing above all. The night before the caucus, the campaign boasted of 4,200 volunteers working to turn people out to vote — and said her team knocked on 125,000 doors.
Sanders, meanwhile, captured young voters and the populist spirit of those dissatisfied with the political and economic mainstream. After last summer's Feel the Bern tour of liberal cities, the Vermont senator has turned out hundreds and even thousands with relative ease throughout Iowa.
His appeal has been much starker than Clinton's — where she promises incremental change through political experience across an array of domestic policy areas, Sanders promises "political revolution."
It would have been a staggering upset if Sanders won outright. This time last year few people knew who Sanders — who refused to label himself a Democrat — even was.
And although Sanders is winning big time in the polls in New Hampshire, which follows Iowa, Clinton will likely perform significantly better after the voting moves beyond the first two states and into more diverse states like South Carolina (and others in the South) and Nevada.
And here's the rundown for the Republicans:
Texas Sen. Ted Cruz solidly defeated businessman Donald Trump and Florida Sen. Marco Rubio, defying polls going into the contest that showed Trump with a sizeable lead.
Cruz appeared to have faded a bit down the stretch to Iowa, under attack from Trump and a series of super PAC ads that questioned how sincere a Christian he really is. Still, voters turned out for Cruz in force.
"Tonight is a victory for courageous conservatives," Cruz said in his speech. "Tonight is a victory for millions of Americans who shouldered the burden of seven years of Washington deals run amok."
"Morning is coming, morning is coming," Cruz added. "When the media in one voice said a conservative cannot win ... over 800,000 contributions poured in to tedcruz.org as courageous conservatives said 'yes, we can.'"
The biggest story of the GOP race, though, was Florida Sen. Marco Rubio's surge to compete with Trump for second place. Trump led Rubio widely in the polls.
Rubio called his showing a victory, outperforming expectations as his support in the stage surged late last week.
Rubio's strong showing in Iowa puts him in a three-man race with Cruz and Trump as the candidates head into New Hampshire and South Carolina in the coming weeks.
"So this is the moment they said would never happen," Rubio said in his speech, which came before Trump made his. "For months they told us we had no chance. For months they told us because we offered too much optimism ... we had no chance."
"They told me that we have no chance because my hair wasn't gray enough and my boots were too high," Rubio said, a reference to the media reports about his fashionable footwear.
Trump appeared boisterous in his very short speech, saying, "I absolutely love the people of Iowa."
"We love New Hampshire, we love South Carolina," Trump said. "We will go on to get the Republican nomination."
He finished, "I think I may come here and buy a farm."
Trump's second-place result led a lot of people to share a 2013 tweet:
BuzzFeed News correspondent Evan McMorris-Santoro reported from the lively Sanders party. Here's some of the scene:
When Hillary Clinton's voice played through the speakers at Bernie Sanders's caucus night party here Monday night, the crowd booed. When glitches caused the video of Clinton's caucus night speech to freeze, the crowd cheered.
When Clinton could be heard saying "I am a progressive," the crowd lost its collective mind with groans and boos.
Also, they played the Rocky theme:
The Carson campaign had by far the strangest moment of the caucuses when it abruptly announced that the candidate was leaving Iowa in the evening to return to Florida — because he needed fresh clothes.
"You know what it's like being on the road campaigning for weeks?" Carson advisor Armstrong Williams said in a phone interview, "Don't you need to refresh sometimes?"
Williams also asked if the reporters sometimes needed to get themselves fresh clothes.
There have been conflicting reports about why Carson — whose stock has plummeted in recent months as a candidate, and whose campaign has burned through millions of dollars as top officials have quit — was leaving Iowa.
Some reported that Carson was going home to rest and would not travel to New Hampshire and South Carolina, while others suggested he was going home to beat a storm and would maintain his campaign schedule.
Read more here.
Here's a little look inside a caucus from BuzzFeed reporter Rosie Gray in three tweets. She was at Stowe Elementary School in Des Moines. They went for Trump.
Turnout for O'Malley — who announced he was suspending his campaign more than halfway through the voting — was dire. Here was the scene at one of his caucus locations:
The first O'Malley backer turned up: a local attorney called Megan Roos.
"I'm not that well-educated a voter," she said when asked why she was voting for O'Malley. "But I do like him. I'm not a socialist and I'm not a Republican so he's kinda my candidate by default. I have met him."
And let's end with these moments of electoral zen. First, even though a presidential race is underway, sometimes you still need to get that workout in:
Here's more coverage from the BuzzFeed News team:
Correction: An earlier version of this post said the Sanders campaign was returning to Florida after Iowa. It is the Carson campaign.