Join us for live coverage of the Iowa caucuses.

Voters in Iowa will cast the first ballots of the 2016 presidential election in caucuses across the state on Monday. Polls show a close race in both Democratic and Republican contests.

Hillary Clinton is leading Bernie Sanders by around three percentage points, and Donald Trump is ahead of Ted Cruz by a more comfortable eight-point margin. Marco Rubio holds a strong third place in the Republican race at around 15 percent. Here’s a guide to understanding the results.

Watch the Turnout

Polling, of course, is not perfect. It’s more challenging in primaries than in general elections, because there are far more undecided voters and turnout is more unpredictable. It’s even harder in an early caucus; the turnout is especially variable and the views of voters are unsettled between many more candidates than remain in later states.

It would not be particularly surprising if either Mr. Cruz or Mr. Sanders went on to win tonight. Turnout is a critical question in both races, and polls always struggle to anticipate the composition of the electorate. Polls show Mr. Cruz and Mrs. Clinton faring well among traditional caucus-goers, and Mr. Sanders and Mr. Trump faring best among those with no prior participation.