Utilizing a complicated in-person caucus system, the first contest in the Democratic presidential primary will take place in Iowa on Monday.
Iowans do not use paper ballots, but instead hold meetings across the state to pick their choice for president.
There are 11 Democrats running for their party's nomination to challenge Republican President Donald Trump in the November election.
Up for grabs will be 41 delegates to the Democratic National Convention, where a candidate will need to secure 1,991 delegates to become the party's nominee.
HOW DOES THE PROCESS WORK?
Instead of heading to a polling site to cast their ballots, Iowans conduct a series of caucus meetings across the state, gathering in school cafeterias, community centers and private living rooms to tally who they think should be their party's nominee.
In total, Iowans hold more than 1,600 caucus meetings. The results are sent to the state party, which then calculates how many delegates each presidential candidate receives.
In each caucus, voters will divide themselves up and stand with others who are supporting the same candidate. If there are enough like-minded voters, then their candidate gets a delegate, referred to as being "viable."
However, voters who pick a candidate who does not have enough support - often at least 15% of those attending the caucus - will be forced to make a second choice.
They will have three options: back a candidate who is already viable, combine forces with the supporters of another unviable candidate and make them viable, or leave. This process is known as "realignment."
The meetings are held at the same time and are scheduled to begin at 8 p.m. ET on Monday (0100 GMT Tuesday)
WHY SECOND CHOICES MATTER
Which candidate is a voter's second-choice preference can play a big role in the outcome.
Currently, most opinion polls show only four candidates — former Vice President Joe Biden, former South Bend, Indiana Mayor Pete Buttigieg and US Senators Bernie Sanders and Elizabeth Warren — registering above 15% in Iowa.
Since a candidate must have at least 15% of the support of those in attendance in any given precinct in order to remain "viable," supporters of other candidates will be forced to realign or pick someone else.
Collectively, the four front-runners' supporters make up just above 70% of the electorate, according to national polling average.
That leaves about 30% of voters who could be forced to pick their second choice among those four candidates - a large number that could tip one of the front-runners ahead of their rivals.
For example, if US Senator Amy Klobuchar is unlikely to reach viability in most precincts, are her supporters more inclined to back another woman, like Warren, or a candidate closely aligned with her ideology, like Biden or Buttigieg?
WHAT IS DIFFERENT THIS YEAR?
The state party is changing the way that results are reported this year, after complaints from supporters of Sanders in 2016 who narrowly lost Iowa to Hillary Clinton, the eventual Democratic nominee.
Previously, Iowa Democratic officials only made public the final results of the caucus, or how much support each candidate after realignment had forced people to make their second choice.
In 2016, Sanders supporters argued that on the first count he had more support than Clinton. But that after realignment, when most supporters of former Maryland Governor Martin O'Malley were forced to make a second choice, then Clinton had more support.
To mollify that complaint, this year Iowa Democrats will make public both counts — how voters align on their first choice and then the results after supporters of unviable candidates make a second choice.
Additionally, for the first time, candidates are required have at least 15% support to be considered viable or to win a delegate. In previous years, the number was calculated based on attendees and available delegates.
The new floor is most likely to impact those candidates whose support is around 10%, a level of backing that might have been enough to win a delegate in past, but could leave a candidate with no delegates this year.
WHY IOWA MATTERS
In the grand scheme of things, the size of Iowa means they have little influence on the final delegate count for the Democratic nominee.
Iowa has 41 delegates out of the 1,991 delegates needed to secure the nomination. Texas, by comparison, has 228, while California has 415. (Graphic: here)
Add to that Iowa is a predominately white state, with minorities accounting for less than 10% of the population. By comparison, about 27% of the US population is non-white.
But Iowa exerts outsized influence by going first. Historically, a field of candidates narrows to the top three or four after Iowa.
And the state has a pretty good track record of picking the Democratic nominee. In the past five contested Democratic primaries, the winner of the Iowa caucuses has gone on to be the party's nominee.
In the 12 Democratic caucuses that have been held in Iowa since 1972, nine times the top delegate winner in the state has ultimately been the nominee. Iowa had less success in picking Republican nominees. The past three Republican winners have failed to secure the nomination.