It's been an election that has in many way re-defined American politics and the way campaigns are run. So when all things are said and done, what are the major storylines you should watch out for tonight as the results are pouring in?
1. High Turnout Bodes Well for Bernie Sanders and Donald Trump
The big question surrounding the Republican race is whether Donald Trump's supporters will show up to caucus or avoid the process altogether. In a caucus, there's no absentee ballots or a full business day dedicated to voting, allowing Americans to head to the polls before, during, or after work. You have to physically go to a location at 7 p.m. CT in your neighborhood, even if the temperatures are cold and a snow storm is looming. Caucuses can also last two hours.
With a strong ground game, offices all over the state, and more than 10,000 volunteers, Bernie Sanders can expect to have his support out in force. But where they caucus and if that voting is too concentrated in certain areas is the big question.
The key for both Sanders and Trump is motivating the first-time caucus-goers they've been able to bring to their Iowa events to come out in large numbers, similar to what happened in 2008, when turnout doubled.
2. Where Will Romney 2012 Voters Turn?
Mitt Romney, an establishment Republican, nearly won in Iowa in 2012, losing to Rick Santorum by only 34 votes, receiving 25 percent of the vote in Iowa. But it remains to be seen which 2016 candidate will get the votes of the type of people who supported Romney.
The candidates who hold the most similar policy positions to Romney are John Kasich, Jeb Bush, and Chris Christie, who are all polling in the low single digits in Iowa. All three are spending caucus night in New Hampshire, where they're polling higher. Some Iowa Republican strategists say that 25 percent could be split between Marco Rubio and Donald Trump, but that segment of voters could be key to boosting Trump over Ted Cruz.
3. Poor Iowa Performances and the Dropout Effect
Rick Santorum and Mike Huckabee, both past winners in Iowa, have not picked up any traction in 2016, even after they both made the state a centerpiece of their campaigns. With both of them low on funding, it will be difficult for either to continue on without placing in the top three.
In late December, on Iowa's WHO radio, Huckabee said: "If we can't come within striking distance of the victory or win it, then I think we recognize that it's going to be hard to take that on to the other states."
But Sunday night at a house party in Iowa, Santorum told reporters he has "no intention of dropping out of the race."
On the Democratic side, Martin O'Malley has held 187 events in Iowa, but has not been able to go above 4 percent in the polls and could call it quits if things don't go his way.
4. What a Trump Victory Means for the Future of the Caucuses
When Donald Trump's helicopter flew over the Iowa State Fair back in August as other candidates like Hillary Clinton and Rick Santorum looked up in the sky, the billionaire businessman was letting everyone know his plans to run a campaign his way.
Instead of riding a bus or RV around Iowa's 99 counties meeting voters in coffee shops and pizza ranches, Trump opted for flying in and out on his Boeing 757. Three-day trips or more than one event a day were rare for Trump, but somehow he has managed to hold frontrunner status since the summer months.
Trump's campaign is built around his unpredictable rallies, featuring controversial remarks to not just hundreds, but thousands. He's also been able to appeal to almost every faction of Iowa voters, not just Republicans but independents and even some Democrats.
5. The Importance of Being First in the Nation and Picking 'Winners'
Donald Trump often tells his crowds at Iowa rallies that "you haven't been good at picking winners. We've got to pick a winner this time." Trump's referring to the state's Republican voters picking candidates who don't make it past the other early voting states, failing to secure the nomination.
Some say Iowa's role is to narrow the field rather than pick the nominee, but this has allowed other states like New Hampshire, South Carolina, and Nevada to criticize Iowa for its inability to catapult someone who represents the sentiments of the country.
But in 2008 Iowa's Democratic voters gave Barack Obama the boost he needed to beat Hillary Clinton and win the nomination. Bernie Sanders is hoping the same thing happens in 2016.