Marianna Brady
BBC News, Des Moines

“Everybody get your calculators out”

They’ve counted, and counted again. There are exactly 240 people here at the Des Moines Community Playhouse.

Nothing is more important than getting the number of people in the room right tonight – because that determines the number of voters needed for a candidate to be viable. Anyone with under 15% is not considered.

15% of the 240 is 36, so in our precinct, a candidate needs 36 supporters for viability.

They're now having to separate the Warren and Sanders supporters - there are so many of them.

The two lone Tulsi Gabbard supporters behind me look sad.

Anthony Zurcher
BBC North America reporter, Iowa

At each of the 1,677 locations across the state, a candidate must have the support of at least 15% of voters in order to qualify for a chance to win Iowa delegates to the national convention in July - when the candidate is ultimately picked.

After a first round of tabulations, those who support a candidate that doesn't reach "viability" have the opportunity to switch sides.

Complicating everything is the new reporting system the Democratic Party has instituted for the Iowa caucus results.

In the past, the party has only released one number - the final tabulation of support after all the caucus horse-trading and support swapping takes place. This time, however, there will be two sets of numbers - the final tally as well as a count of each candidate's support in the first round of balloting.

That means two - or more! - candidates could be on stage declaring victory on Monday night.

It has the makings for a long, chaotic night - the first of what could be many for the candidates still standing after the Iowa dust settles.

Read more from Anthony here.

The BBC has teams across caucus rooms in Iowa tonight, tracking how voters feel about their preferred candidate to take on Trump.

Stay with us for all the live results as they come in.

Marianna Brady
BBC News, Des Moines

Tim Ingram, 60

I’m not really undecided - I was supporting Cory Booker. I am going to stand with Cory Booker and try and make him viable, even though he stopped his campaign.

If that doesn’t work, my plan is to go to Amy Klobuchar to see if she’s viable. If she is, I’ll caucus for Andrew Yang because I think it’s important to make him viable.

Samantha Worth, 21

I’m leaning towards Bernie but I’ve never caucused before and it’s a cool opportunity to hear all perspectives - so I want to make the right decision before I put my full support behind a candidate.

Andrew Lafont, 26

I’m undecided because I haven’t seen anyone that I really like so far. I’m pretty much here because I had nothing else to do tonight and I didn’t want to sit alone at home all night.

Out of all the candidates Bernie has the biggest shot of winning the Democratic nomination, but I’m not sure he could win the general. I didn’t even know who Amy Klobuchar was until I came here tonight.

Marianna Brady
BBC News, Des Moines

We're at the Des Moines Community Playhouse and the caucus is about to begin. An estimated 237 Democrats from the neighbourhood have been filing in for the past hour - many greeting one another with friendly hellos. The theater, which has the stage set up for The Diary of Anne Frank, has been turned into a mini political convention for the evening.

Volunteers for the main candidates have blocked off portions of the theater with signs and banners. Many attendees that have walked in have gone straight to the candidate they know they want to support, but there is a small contingent of undecided voters in the far corner of the theater.

As it stands, the Elizabeth Warren and Bernie Sanders supporters fill up most of the theatre.

It's going to be a complicated process, so our Marianna Brady (who is in a caucus in Des Moines) will pop up soon to explain how they work.

So roughly, here's what to expect:

• 19:00 local time (20:00EST) - caucuses open
• 20:00 local, roughly: results of first round of voting expected
• ?? - results of second round of voting (once candidates who get less than 15% are eliminated)
• ?? - the announcement of how many delegates each candidate gets

Hopefully that helps.

Image source, Getty Images

You can thank Jimmy Carter – sort of.

In 1972, Iowa became the first state to hold its caucuses thanks to a number of boring technical reasons, including the sheer number of electoral processes they had to get through.

But when Carter ran for president in 1976, his team realised they could grab the momentum by campaigning early in Iowa.

He won there, then surprisingly won the presidency, and Iowa's fate was sealed.

As political analysts have joked: Iowa is not first because it’s important, but it's important because it’s first.

Image source, Getty Images

Who? The 2016 runner-up and Vermont senator needs little introduction

Key issues: Medicare-for-All universal healthcare coverage; raising taxes on the wealthiest Americans; upping the minimum wage

One policy: Completely eliminating $1.6tn in existing student debt - regardless of income, unlike his rival Elizabeth Warren - and making public colleges, universities and trade schools tuition-free by taxing Wall Street

Read more, including analysis from the BBC’s Anthony Zurcher.

A win here for any candidate can help give them momentum and propel them to victory in the primaries. If a candidate has struggled here, they may find donations drying up, as they're already on the back foot in the race for the White House.

But there are also plenty of reasons Iowa doesn't matter.

Iowa doesn't represent the entire US - it's largely white, so the way people vote here will be very, very different than in other states (like Nevada and South Carolina later this month).

Its record on picking the eventual nominees is a bit rubbish too, at least when it comes to Republicans - when there's an open Republican race (unlike this year), Iowa hasn't opted for the eventual nominee since 2000. Such names as Mike Huckabee, Rick Santorum and Ted Cruz have won there in recent years.

Our North America editor Jon Sopel and his team are at the Bernie Sanders campaign event in downtown Des Moines (they're quite early).

It's a bit quiet there right now, but that might all change later.

Dan McAdam
BBC Radio 4 Today, Iowa

In the Iowan city of Newton, a group of Democrat caucus-goers reel off all the presidential hopefuls they’ve seen up close.

"I saw Buttigieg, I saw Booker, I saw Biden, I saw Sanders, but I haven't 100% made my mind up,” says Josina.

"When I saw Joe Biden yesterday, that really sealed my thinking,” says John, who’s lived in Newton for 20 years. “I like Buttigieg, I like Klobuchar, I like a lot of the other candidates. But to see them in person, see them up close, see how they react to questions can really help you sort out who you're backing."

There’s a reason all the Democratic hopefuls have stopped here. President Trump won this area, Jasper County, from the Democrats in 2016. Newton, a city of about 20,000 people, is a former manufacturing hub, and has suffered since the washing machine manufacturer Maytag, which was the heart of the town, closed operations.

Image source, Getty Images

Gezellig Bar, where we spoke to Democratic voters, sits on the campus of the former Maytag headquarters and is next door to where most of the candidates stopped to speak.

Those who haven’t made their mind up yet can expect to get a fair amount of attention in tonight’s vote. “If you're not 100% committed to your candidate you will be approached by all of your neighbours trying to get you swayed to their candidate.

"It can be a little bit frenzied and crazy like everyone wants you to be their date to the prom," says Josina.

Image source, Getty Images

Who? Former vice-president and veteran senator

Key issues: Rebuilding the middle class; investing in federal infrastructure; tuition-free public universities

One policy: Similar to the Green New Deal, Biden's Clean Energy Revolution would make the US economy 100% clean energy based with net-zero emissions by 2050, as well as target polluters with fees and quotas

Read more, including analysis from the BBC’s Anthony Zurcher.

Helier Cheung
BBC News, Washington DC

It’s been a hectic couple of weeks for Bret Nilles, chair of the Linn County Democrats.

There are 51,000 registered Democrats in Linn County, and he’s responsible for organising caucuses in all 86 precincts. He’s likened it to “planning 86 wedding receptions when you don’t know how many people are going to show up”.

However, he thinks the caucuses are also important because they give voters a real stake in events.

Voters are “willing to come out on a winter night in February and stay there for two to three hours - that shows a level of engagement that I don’t think you see in many other areas,” he told the BBC.

The Iowa caucuses will be starting in an hour - we won't get any sense of the results until about an hour after that. And even then, they won't be the final, final results...

Zhaoyin Feng
US correspondent, BBC Chinese

In every one of Andrew Yang’s rallies that I have been to, I have seen a large turnout of Asian-American voters, who generally tend to be less politically active. But Yang, one of the most recognisable Asian-American presidential candidates in history, has successfully mobilised this group.

Sixteen-year-old Katerina Li, a Chinese-Canadian living in Iowa, stared at Yang in starstruck awe during his last pre-caucus rally in Des Moines. She said she was impressed by Yang’s charisma and encouraged to see an Asian running for president. “Representation matters. I hope to see more diversity in American politics.”

The shared ethnicity is what initially drew many Asians into the “Yang Gang”, but most of them say that after they learned more about his platform, they became more convinced that Yang was their candidate.

Image source, Reuters

Chinese-American Yang Deng got hooked after watching Yang’s speech videos for hours. She traveled from California to Iowa the weekend leading up to the caucuses to canvas for Yang. Though familiar with Yang’s signature policy, a universal basic income of $1,000 a month for every American, Deng said her favourite policy proposal was the “Democracy Dollars” – every American gets $100 a year for political donation.

“This will wash off the corporation dollars and the mega donors. The government will be on people’s hands,” Deng told me.

According to filings with Federal Election Commission, Yang has become the top recipient of Asian-American donations among the Democratic presidential hopefuls.

On the Chinese messaging app WeChat, more than a hundred Yang-supporting Chinese Iowans are exchanging caucus-related information, including a photo of a bespoke fortune cookie message – “Good fortune to you and to America. Vote for Andrew Yang.”

They’ll be caucusing for Yang tonight, hoping the presidential candidate who looks like them will surge.

There are 11 Democrats still standing (from 28 who have stood at one point or another), though realistically not all of them will do well tonight.

Here's who is left:

• Bernie Sanders, Vermont senator
• Elizabeth Warren, Massachusetts senator
• Joe Biden, former US vice-president
• Pete Buttigieg, former Indiana mayor
• Amy Klobuchar, Minnesota senator
• Michael Bloomberg, former New York City mayor
• Tulsi Gabbard, Hawaii congresswoman

• Andrew Yang, tech entrepreneur
• Tom Steyer, billionaire investor
• Deval Patrick, former Massachusetts governor
• Michael Bennet, Colorado senator

We'll bring you potted profiles of the candidates - as many as we can - over the course of the next few hours.

David Grossman
BBC Newsnight, Des Moines

Who doesn’t want to meet the next president of the United States? Obviously, there are plenty of unknowns, not least the fact that the current president is intent on staying in the Oval Office. But Iowa is where journalists can get close - really close - to the people who want to run America.

Image source, Getty Images

The state is mainly rural and sparsely populated so campaigning in Iowa means organising three or four small town hall style meetings every day. The key to winning is shaking hands and posing for photos. That means they are in range of my microphone. I pick the events in more isolated areas away from the state capital Des Moines. And then you have to know exactly where to lurk to grab a chat.

After next month the candidates will have Secret Service protection and will be campaigning at fewer, bigger events, so this reporter’s paradise doesn’t last long.

Generally, primaries are open to more voters - they’re held across the state, while caucuses are held in just a few places at a particular time, and are open only to party members. Caucuses used to be the most popular way of choosing candidates, but they’re much less common now.

In primaries (like the first, in New Hampshire next Tuesday) you vote in private - in caucuses, it's often through a head-count.

Anthony Zurcher
BBC North America reporter, Iowa

At an Elizabeth Warren rally on Saturday in Iowa City, before the candidate appears on stage to speak, Nina Elkaldi takes the microphone and makes a promise to the crowd. The Iowa Caucus campaign season is almost over, and soon the state’s residents would be getting some peace and quiet.

“In just two days we will no longer be receiving 39 texts, 40 phone calls and eight mailers every single day,” the campaign volunteer, who is taking time off from college to serve as a caucus precinct captain, says.

The crowd laughs. They know.

Maeve Jackowski knows. She works at the nearby University of Iowa hospital and only recently decided to back Warren in the caucuses. She says she’s been inundated by messages from all the Democratic campaigns. She sometimes gets five different volunteers knocking on her door in one day.

“To be honest, some days you just kind of keep your blinds shut and you can't do it,” she says. “Sometimes you’re, like, my dog is going crazy. I can't have any more visitors."

Both Jackowski and Elkaldi acknowledge that living in the state that starts the presidential nomination process is a burden sometimes – but they also say it’s a privilege they and their fellow Iowans take seriously.

“The caucus process itself is unique to Iowa obviously,” Elkaldi says. “And I think it's actually really exciting. It's democracy at work. Neighbours are talking to each other. Instead of going to that ballot box, you're getting the chance to hear why people are supporting their candidate. I think it's really special.”

Oddly, by the time you’re reading this, some of the Iowa caucuses have already taken place. Even more oddly, one of them was in Georgia (the country, not the state).

That’s because Iowans were allowed to hold “satellite caucuses” around the world this year. Eager Iowans unable to make it home held caucuses in Paris, France, Glasgow in Scotland, and in Tbilisi, the capital of Georgia.

Journalist Joshua Kucera is the man behind the caucus in the Caucuses, external (as an article he wrote in The Nation wittily called it). He found a grand total of two other Iowans in his adopted country, and all three voted in his apartment.

“Like most Iowans who have left, I’m nostalgic for home,” he wrote. “And one of the many things I love, even when it disenfranchises me, is our archaic voting system.”

If you're interested, Bernie Sanders came out on top in Glasgow, and Elizabeth Warren in Paris. The Tbilisi result isn't yet clear, not that it will make much difference...