Katty Kay
Des Moines

This is my fifth Iowa caucus and the overwhelming sense I get this year is nervousness.

Sometimes you get excitement - 2008 - sometimes there’s anger - 2004 - but this time there’s just anxiety.

In the two days I’ve been here I’ve spoken to three candidates, and been to two rallies. Everyone I spoke to knows it won’t be easy unseating Mr Trump. They hope these caucuses will give clarity and unity to a divided field.

Iowa's caucuses begin at 19:00 local time (20:00 EST) and should last about an hour. There will be caucuses in 1,678 locations (including gyms, libraries, and even private homes) across Iowa’s 99 counties.

Image source, Getty Images

Anyone can vote, as long as they’re a registered voter. The process is a bit confusing, but it works like this:

• you stand in a certain part of the room to show which candidate you support
• if, at the end of that vote, any candidate gets less than 15%, they then become invalid
• the people who voted for them get to pick someone else (or not vote at all)
• only at this point will we know how many delegates each candidate gets

All the results will be made public. It's entirely feasible that the person who does best in the first round doesn't win the second. This is because the candidate in second place in the first round may scoop up more of the "other" votes.

Hopefully that makes sense...

Or maybe it's like the Oscars? The analogies are not that far off, as our Marianna Brady explains...

The US primary process is how each party’s presidential hopefuls battle for the presidential nomination. There are primary elections, similar to a general election (where you vote in private), and there are a handful of caucuses.

Caucuses are party-run events, where people gather at venues across the state (like barns or churches instead of polling stations) to vote on their preferred candidate – sometimes by head count or show of hands.

As a result, caucuses tend to really suit candidates who are good at rousing their supporters to get out of bed. People like Bernie Sanders, for example.

Caucuses used to be far more popular back in the day, but this year, Democrats are holding only four in US states - in Nevada, North Dakota, Wyoming and Iowa.

The more votes a candidate gets in a caucus, the more delegates they are awarded. Each of those delegates then votes for them as the party’s candidate in the summer convention.

Anthony Zurcher
BBC North America reporter

Image source, Getty Images

Iowa doesn't always determine the ultimate Democratic nominee, but when it's a wide open field like this - without a clear home-state favourite - it can be a pretty accurate predictor.

Hillary Clinton narrowly won in 2016. Barack Obama in 2008, John Kerry in 2004 and Al Gore in 2000 all had more comfortable victories. Every single one went on to become their party's standard-bearer.

What's more, an Iowa win followed by one the following week in New Hampshire is a seldom-accomplished feat. Only two non-incumbent Democrats, Kerry and Gore, have done so since the modern primary system began.

If polls are any indication, Bernie Sanders has the potential to pull off such a double play. A Bernie boom scenario starts in Iowa.

• Read more from Anthony here

Nine months from today, the US will vote on their next president.

It's all but certain that Donald Trump will be the Republican nominee. But the race to decide who will run against him starts today.

The Iowa caucuses take place from 19:00 local time (that's in a little under three hours), and while victory there will not guarantee anyone the Democratic nomination, it can really help bring momentum to a candidate if they win there.

Stick with us over the next few hours - we'll bring you all the latest updates, context and trivia from our team on the ground in Iowa, and here in Washington DC.