Election Day is underway, with tens of millions of voters heading to the polls around the U.S. But it could be days, or even weeks, before it becomes clear which party will control each chamber of Congress for the remainder of President Donald Trump’s second term.

Some races are likely to be called on Tuesday night, while in other, tighter contests it could take multiple days—if not longer—to determine a winner. It all boils down to the states, which the U.S. Constitution empowers to establish the times, places, and manners of holding elections and counting ballots.

Certain states are known for having shorter count times, such as Florida. Others typically take longer, like California, due to its size, and Alaska, where ranked-choice voting can make the process of certifying votes more complicated.

“There are all these differences, both in process and deadlines, and geography, and who the voters are, and who the candidates are, that make a difference in what this looks like,” Mitchell Brown, a professor of political science at Auburn University and the author of “How We Vote: Innovations in American Elections,” tells TIME.

Mitchell says that it could take up to 30 days before all the states certify their results. She stresses that official midterm results released by the states themselves are different from projections from media outlets, which will call races sooner.

“It's really important for everyone to understand that what happens on election night when we're watching TV or looking online, and we're seeing races called, those are predictions by media outlets. They're not the official results,” adds Chris Mann, the research director at The Center for Election Innovation & Research. “With close elections, we may end up not knowing who the winner is until we get to those official certifications that every vote has been counted.”Here’s what to know about how the midterm votes will be tabulated, and when we are likely to know the results. When do the polls close?

Polls open and close at varying times in different states around the country, with most closing their polls around 7 p.m. or 8 p.m. local time. Because the U.S. is divided across several time zones, however, those closures are spread over several hours on Election Day. And in some cases closing times vary even between different parts of the same state.

Here is when the latest polls close in each of the 50 states and Washington, D.C., in Eastern Standard Time.

7 p.m. EST: Georgia, Indiana, Kentucky, South Carolina, Vermont, and Virginia.

7:30 p.m. EST: Ohio and West Virginia.

North Carolina also typically closes its final polls at this time, but due to minor technical issues reported in some localities on Tuesday—which have since been resolved—the state’s board of elections has extended voting hours in some precincts.

8 p.m. EST: Alabama, Connecticut, Delaware, Florida, Illinois, Maine, Maryland, Massachusetts, Mississippi, Missouri, New Hampshire, New Jersey, Oklahoma, Rhode Island, Tennessee, and Washington, D.C.

In Pennsylvania, most polling sites will close at 8 p.m., but in three precincts they will stay open for an additional half hour “due to machine set-up issues” that delayed the opening of those polls this morning, according to Chester County officials. 8:30 p.m. EST: Arkansas. 9 p.m. EST: Arizona, Colorado, Iowa, Kansas, Louisiana, Michigan, Minnesota, Nebraska, New Mexico, New York, North Dakota, South Dakota, Texas, Wisconsin, and Wyoming.

10 p.m. EST: Montana, Nevada, and Utah. 11 p.m. EST: California, Idaho, Oregon, and Washington state. Midnight EST: Hawaii. 1 a.m.

EST: Alaska How are ballots counted? After their respective polls close, states must verify and count ballots that were cast in various ways, including those made in person during the early voting period and on Election Day, mail-in ballots, and provisional ballots.

Depending on the state, ballots are either counted right at voting locations, or transported back to county counting facilities. Mann says these facilities are like warehouses containing machines that can quickly and accurately count votes.

Provisional ballots allow people to vote who fail to meet certain eligibility requirements that can later be verified. If, for example, someone arrives at a polling site and does not have their ID, they may be able to vote through a provisional ballot that can then be counted if and when they have verified their identity.

Brown says the process of confirming provisional ballots is called “curing” and can add additional time to the tallying process.

Fourteen states and Washington D.C. also allow ballots postmarked by Election Day to be counted if they are received during a set post-election window, which can also add delays to final vote counts. Why does it take longer to get results from some states than others?

It will take significantly longer for the results of the election to be officially tabulated—or even projected by media outlets—in some states than in others.

When it comes to the media calling races, projections may take longer to come in cases when races are tight, and the winner does not become clear until nearly all the votes are counted.

“If there is a high margin of victory, they can make that projection sooner,” Wren Orey, who runs the Elections Project at the Bipartisan Policy Center, a policy think tank, tells TIME. “Whenever there's a very narrow margin of victory, that means that election officials need to process and report more results before a media outlet can be confident projecting a winner.”

In terms of the actual process of states counting votes, meanwhile, differences in speed can boil down to physical factors, such as the distance ballots must be transported before being counted. In states such as Alaska, for instance, vast and rural territories create delays for counting ballots. “There are sometimes ballots being flown back to Anchorage in order to be processed,” says Orey. They add that some states may also simply have more infrastructure to count ballots, including more machines, employees, and counting locations.

State laws governing how votes are cast and counted can also play a role in the timeline of election results. Alaska and Maine, for example, use ranked-choice voting, a process in which voters rank their top candidates instead of choosing one person for a position. Mann describes this method as a “more complicated process” in tight races, where ballots may need to be recounted due to one candidate not receiving an outright majority in the first or even subsequent rounds of counting.

“If no candidate has a majority on that first count, then they drop the candidate with the lowest number of votes, and they go through it again,” Mann explains.

Other states, like Wisconsin and Pennsylvania, prohibit preparing mailed ballots for scanning until Election Day.

“Those are two of several states that don't start this process until the morning of Election Day, and they will have hundreds of thousands of ballots,” to prepare to be counted while running the polls at the same time, Mann says.

Fraud is extremely rare The process of confirming election results taking days or weeks, Mitchell says, is completely standard, and is not an indication of election fraud, which research and state audits have consistently shown to be extraordinarily rare.

“The thing that I would hammer home is not knowing an official result the day of the election, the day after the election, even a week after the election, is not evidence of fraud,” says Mitchell. “And to say so is to misconstrue and manipulate people.”

“[Fraud] is extremely rare … Elections are more secure, more accurate than they have ever been,” adds Mann. “Election officials and your friends and neighbors who are out working the polls are working incredibly hard. They are very dedicated to make sure the election is safe, secure, accurate.”