10:49 p.m. Former Arkansas Gov. Mike Huckabee announced on Twitter that he is suspending his presidential campaign.
10:25 p.m. CBS News projects that Cruz has won the GOP Iowa caucus.
10:17 p.m. Forty percent of GOP caucus goers said they were looking for a candidate who shared their values. Twenty-one percent said electability was the most important quality and 21 percent said it's someone who can bring needed change.
Cruz is winning the values voters, Rubio is getting the support of voters looking for electability, and Trump wins among those who said they want a candidate who can bring needed change.
10:00 p.m. CBS News reports that the Republican race leans toward Cruz and the Democratic race leans toward Clinton.
CBS is not projecting a winner at this time.
9:58 p.m. CBS News' Hannah Fraser-Chanpong confirms O'Malley is suspending his campaign tonight, according to sources close to his campaign.
9:39 p.m.With half of the vote in on the GOP side, Cruz leads with 29 percent of the vote, Trump behind with 26 percent and Rubio in third with 21 percent.
9:38 p.m. CBS News estimates that Rubio will finish in the top three in Iowa.
9:34 p.m. With just over half of the vote in on the Democratic side, Clinton leads Sanders 51 percent to 48 percent.
9:22 p.m. With a quarter of the vote in for Republicans, Cruz is leading with 30 percent of the vote, Trump is on his heels with 27 percent and Rubio is in third with 19 percent.
9:14 p.m.Republican caucus goers were almost evenly split between those who said they want the next president to be someone who has political experience, 45 percent, and those who want someone from outside of politics, 49 percent.
Of those who want someone from outside of politics, 47 percent support Trump. Of those who want someone with political experience 38 percent support Rubio vs. 32 percent for Cruz.
9:12 p.m.Of those looking for someone who can win in November, 75 percent are Clinton supporters. Similarly, 86 percent of those seeking a candidate with experience support Clinton. Voters that say they want someone who cares about people like them support Sanders (75 percent) as well as those who want to elect someone who is honest and trustworthy (82 percent).
9:09 p.m. With 25 percent of precincts reporting, Clinton is ahead of Sanders 52 percent to 48 percent. One percent supports O'Malley.
9:02 p.m.Sanders seems to have succeeded in bringing in new voters tonight. Among those who said that this was their first caucus, he leads Clinton 61 percent to 35 percent.
Twenty percent of the Democratic caucus goers said that they think of themselves as independents or "something else." Among those people, Sanders leads 70 percent to 25 percent.
8:53 p.m.A Cruz senior staffer tells CBS News: We're exceeding vote goals in numerous counties across the state, so we feel good. Way too early to put a bunch of weight in entrance polls.
8:49 p.m. For those who say winning in November matters when deciding whom to support, 38 percent said they back Rubio, 26 percent support Trump and 17 percent back Cruz.
8:45 p.m. At Trump's first caucus location stop in West Des Moines, the billionaire told voters that his campaign is "very happy" about what they're hearing as the results pour in.
"I built a great great company, one of the great companies, some of the greatest assets in the world, and I say that not in a braggadocious way, I say it because that's the kind of thinking our country needs," he said. "We're in trouble, but we're going to turn it around, we're going to make this country great again."
8:39 p.m. Entrance polls show 78 percent of caucus goers are identifying themselves as Democrats. Among them, 56 percent said they support Clinton and 37 percent said they're for Sanders.
Twenty-one percent of caucus goers identify as independents. Among them, 27 percent said they back Clinton and 67 percent said they support Sanders.
8:37 p.m.For Democrats, there is a gender gap, but it is not as large as some may have expected. Clinton is leading Sanders among women 55 percent to 41 percent. Sanders is ahead among men 47 percent to 41 percent.
Sanders is doing extremely well among younger people. He is getting the support of 85 percent of those under 30 and 58 percent of those 30 to 45. Clinton is very strong among older people; she is getting the support of 57 percent of those ages 45 to 64 and 70 percent of those over 65.
Sanders' support comes from liberal Democrats. Those who call themselves very liberal favor him over Clinton to 58 percent to 37 percent. Among those who want the next president to pursue more liberal policies than Obama Sanders is favored by 75 percent to 18 percent or Clinton. Clinton supporters say they want someone who can win in November and has experience. Sanders voters say they want someone who cares about people like them and who is honest and trustworthy.
8:32 p.m.On the Republican side: Trump is doing well among less educated caucus attenders. For those with a high school diploma or less, 42 percent are Trump voters as opposed to 26% of those with more than a high school degree.
Cruz is doing well among evangelicals getting 26 percent of the vote, but Carson also has some strength among these voters (12 percent) and that is cutting into Cruz's base. Trump does very well among voters who say they want a president who "Tells it like it is" garnering 68 percent of the voters and among those who want someone who can "Bring needed change"(37 percent).
8:22 p.m. The entrance polls show that 59 percent of first-time caucus-goers have picked Sanders and 38 percent have picked Clinton.
8:14 p.m.On the Republican side, Trump is doing well among caucus attendees who have no college degree. Cruz is doing better among evangelicals, but neurosurgeon Ben Carson also has some strength among these voters and that is cutting into Cruz's base.
Trump does very well among voters who say they want a president who "tells it like it is" and among those who want someone who can "bring needed change." Sen. Marco Rubio, R-Florida, is doing best among those who say they decided who to support in the last week. Trump supporters said they made up their minds more than a month ago.
8:08 p.m. Early entrance polling shows that Sen. Bernie Sanders, I-Vermont, is performing extremely well among young people -- especially those under 30 and those ages 30 to 45. He is also drawing votes from people who label themselves very liberal and who want the next president to pursue more liberal policies than President Obama. He is also doing well among first time caucus goers, some of whom may be the younger voters he is attracting.
8:00 p.m. Businessman Donald Trump is leading among early-arriving GOP caucus-goers, and former Secretary of State Hillary Clinton has an early lead among early-arriving Democratic caucus-goers according to CBS News entrance polling.
Among Iowa Democratic voters, 32 percent say their most important issue is healthcare, 28 percent income inequality, 26 percent say economy and jobs, and 8 percent say terrorism.
Twenty-nine percent of those voters also say that experience is their most important quality, while 26 percent say it is honesty.
7:50 p.m.Iowa voters have begun arriving at their local community centers, churches and gymnasiums to cast their votes in the first-in-the-nation caucus and officially kick off the 2016 election.
The doors to the caucus sites will close at 8 p.m. ET. There are 1,681 different precincts across Iowa, which will meet in about 1,100 spots for Democrats and 900 sites for Republicans.
CBS News will conduct entrance polls with Iowans headed into their caucus sites, and results will be posted in this space. There will be live analysis at the CBS News digital network, CBSN.
The caucus process is simple on the Republican side: Caucus attendees show up, and they can hear messages of support from either one of the candidates, a representative from the campaign, or a supporter. People cast a ballot for their preferred candidate, and national convention delegates are awarded proportionally based on the results.
The Democrats' caucus is more complicated. After voters show up to their precinct site. Then they'll divide into presidential preference groups for the candidates they are supporting. For a candidate to be awarded any delegates out of that precinct, they'll need to be "viable" -- that is, they must have the support of at least 15 percent (or, in some cases, more) of the people in attendance. If a candidate is not viable, their supporters can try to win over other caucus goers to meet the required threshold. Or they can disband and support the viable candidates. Their other option is to remain uncommitted entirely.
Based on the final results of the preference vote, each candidate will receive a proportional number of the county convention delegates, and "state delegate equivalents.
This year, results from both Democratic and Republican precincts will be reported by a new mobile app the parties created through a partnership with Microsoft and Interknowlogy.
CBS News Political Reporters Rebecca Shabad and Rebecca Kaplan, and CBS News poll analysts Stanley Feldman and Melissa Herrmann contributed to this story.