Voters headed to the polls on Tuesday to cast their ballots—but it might take days to know whether Vice President Kamala Harris or former President Donald Trump has clinched the 2024 election.

Several factors influence how long it could take to determine a winner. If the race is tight, it could take days for every ballot to be counted; last-minute legal challenges, mechanical failures and weather emergencies could prolong results further.

Here’s what you need to know.

Will we learn the results on election night?

It will depend on how close the election gets. In states where one candidate is dominant, races are often called quickly, sometimes as soon as polls close. But the process can take far longer in places where the vote is neck-and-neck.

Polls have shown a tight presidential race in seven key battleground states that are expected to shape this year’s election: Pennsylvania, Michigan, Wisconsin, North Carolina, Georgia, Nevada and Arizona.

The Harris campaign predicted Monday that Georgia, North Carolina and Michigan would deliver near-complete results on Tuesday night, with Pennsylvania, Wisconsin and Arizona reporting partial results. Harris-Walz campaign chair Jen O’Malley Dillon told reporters the campaign expected additional results from Wisconsin and Pennsylvania early Wednesday, with Arizona and Nevada coming in the rest of the week.

“We believe this race is going to be incredibly close, so we may not know the results of this election for several days,” she said.

Counting mail-in-ballots can further prolong results. States have varying rules about when such ballots can be processed; some can begin weeks ahead while others can’t start until Election Day. More than 39 million mail-in ballots have been returned, according to the University of Florida’s Election Lab.

Recounts can also delay the results, though they tend to be conducted quickly.

When will states begin reporting election results?

Polls close at different times across the country, ranging from 6 p.m. ET in parts of Indiana and Kentucky to 1 a.m. ET in part of Alaska. Results will come in waves as states wrap up voting. The Associated Press made its first race calls—Kentucky and Indiana for Trump, and Vermont for Harris—shortly after those states fully closed their polls at 7 p.m. Several more have since come in.

In 2020, the Associated Press called its first two states, Kentucky and Vermont, right at 7 p.m. Three more came in before 8 p.m. By midnight, over two dozen more states and the District of Columbia had a winner, though the election wasn’t called for several more days.

When have previous winners been announced?

The 2020 race wasn’t called until the Saturday after Election Day. It took days to get the tallies from Pennsylvania, which gave Biden the Electoral College votes he needed to claim victory.

Results came much quicker in the preceding few presidential contests. The 2016 election was called for Trump at 2:29 a.m. the morning after the election; former President Barack Obama ’s 2012 and 2008 wins were both called on election night.

How will The Wall Street Journal determine election results?

The Journal, like many outlets, relies on vote-counting services from the Associated Press. The AP uses the raw vote count, as well as surveys, statistical modeling, election rules and local expertise when making a call. It doesn’t announce a victor until it is sure there is no plausible path for the losing candidate to overtake the winner.

Officially, the winner will be determined much later. Electors in each state will vote in mid-December based on the state’s popular vote for president, and those votes will be certified on Jan. 6, 2025.