Early voting is already underway in many states, and this election season offers more opportunities than ever for voters to cast a ballot ahead of Election Day. Massachusetts and Minnesota, for example, are offering newly expanded early voting options, and other states are expanding mail balloting.

And for the past 16 years, voters across the country have been casting more and more early presidential ballots. According to data from the AP Election Research Group as reported by CBS News, only 16 percent of the votes for president were cast early in 2000, but by 2012, that number had risen to 36 percent. Since 1996, according to data from the Census Bureau, Americans have reported a threefold increase in alternative voting methods. This year, 30 to 40 percent of the vote is expected to come from ballots cast before Nov. 8.

Currently, the turnout of early voters in North Carolina is running slightly behind where it was in 2012, according to Michael McDonald, an associate professor of political science at the University of Florida and an expert on early voting. North Carolina’s registered Republicans were voting more than registered Democrats in mail-in early voting, but once in-person early voting began, turnout spiked among registered Democrats. In Iowa, which is currently a tossup state, Democrats are expected to lead in the early vote, according to McDonald.

Here’s an explanation of how early voting works and what it might mean this year.

What is early voting?

Technically, every state allows at least one form of early voting, the absentee ballot, but that’s not really what experts mean by early voting. That’s because 20 states require an excuse to cast an absentee ballot. The accepted excuses vary by state, but usually include being a student enrolled in an out-of-state school, having an illness, injury or disability that prevents you from voting in person, and being out of state on Election Day.

Usually when people talk about early voting, they’re referring to the methods that don’t require excuses, which come in two forms: mail-in and in-person. Mail-in early voting is similar to absentee voting — voters receive a ballot in the mail, fill it out, and mail it in or drop it off before or on Election Day. For in-person early voting, voters go to polling stations during designated days and times to cast their ballots before Election Day. While several states are expanding access to in-person early voting, three states (Ohio, Arizona and Nebraska), all of which have Republican-controlled legislatures, have cut back on early voting opportunities or limited mail ballot collection for the first time in 2016, according to a report from the Brennan Center for Justice.

What are the differences among the states?

The number of states that offer early voting, and the way in which those states participate in early voting, changes frequently. Currently, 37 states and the District of Columbia offer some form of early voting that does not require voters to provide a reason for not waiting until Election Day. Some allow ballots to be dropped off at election offices, while others set up special satellite polling places, and 22 (plus D.C.) allow early voting on weekends, which is often convenient for people who have a hard time leaving work on Election Day.

Three states, Washington, Oregon and Colorado, conduct all elections by mail, sending a ballot to every registered voter, who can then mail it back early. All three states, however, do provide some polling places for those who prefer to vote in person. There isn’t much of a geographic or political split connecting the states that require an excuse for absentee ballots and do not have in-person early voting.

Early voting periods begin between four and 45 days before the election, according to the National Conference of State Legislatures, and typically end shortly before Election Day.

How popular is early voting?

The number of early votes grows every presidential election year, and according to Census Bureau data, and about a third of all voters participated early in 2012. Nearly 8 million early votes have been cast as of this writing, according to McDonald.

Early voting often plays a big role in battleground states. In 2012, 80 percent of the vote was cast early in Colorado, as was 56 percent of the vote in Florida, according to AP election results.

Who votes early?

People who vote early tend to be involved in the political process and prepared to cast their vote days or weeks before Election Day, according to McDonald.

He separates early voters into two groups: those who have to and those who want to. He said the first group is mostly made up of those who are in the military or on the move, usually young voters, who move away from their home state for college or jobs. They tend to vote by absentee ballot.

Brett Neese, 23, a philosophy major at DePaul University in Chicago, cast his ballot in his home state of Iowa on Sept. 30. He said the process was simple.

“At this point, all I see is an opportunity to disengage from the very violent and divisive discourse” of this election, he said. “And there isn’t a whole lot that could change [my mind].”

The second group of early voters are made up of those who are already sure who they’re supporting. McDonald said the people in this group have been closely following the campaigns, are passionate political junkies and aren’t going to change their mind.

As Barry Burden, a University of Wisconsin-Madison professor who studies early voting, said early voters are motivated by “all the factors that make someone a voter in the first place, but just stronger.”

In 2008 and 2012, black voters, who are generally less likely than white and Hispanic voters to cast early ballots, participated in early voting at about the same rate as white voters, presumably at least in part because of President Obama’s presence on the ticket and the success of his get-out-the-vote campaign.

What effect will early voting have on the outcome?

Early voting may have a slight potential to affect the outcome of this election, but experts say its predictive value is not particularly high.

Burden and McDonald agreed that the majority of people who cast their ballots early would have participated in the election anyway and likely would not have changed their minds if they’d waited until Election Day, so the timing of their votes probably won’t change the outcome. Moreover, McDonald said it’s very difficult to identify national trends in early voting, since the laws vary widely by state and different voting opportunities attract different kinds of voters.

In general, though, Democrats who vote early tend to do so in person and Republicans tend to do so by mail. But that isn’t true everywhere — Oregon, Washington and Colorado all offer mail-in ballots to every registered voter, and most of their votes have gone to the Democratic candidates in presidential elections, at least in the last two electoral cycles. Early voting in the past two presidential elections has favored Democrats, McDonald said.

Well-organized campaigns do have opportunities to capitalize on early voting, however, and this year that could benefit Hillary Clinton, who has a stronger ground game than Donald Trump.

It “opens up more possibilities for voting, boosting turnout in the long run,” said Mark Stephenson, the CEO of Red Oak Strategic, a political consulting firm in Arlington, Virginia. “But it also gives the campaign tacticians the opportunity to analyze and see what is happening over a longer period of time and be efficient with where spending is going as a result. Both, when done successfully by either party, can provide a real tactical and strategic advantage.”

“I suppose it probably advantages the Democrats slightly,” Burden said, “but that’s mostly because the Democrats are organized to take on the early vote more than the Republicans are.”

Jon Ralston, a prominent political reporter in Nevada, noted that Clinton can take advantage of the Democratic Party’s edge in organizing early voting, which was built in 2008 and 2012.

The Clinton campaign uses a variety of techniques for reaching out to early voters, including door knocks, phone calls, emails and text messages, said Lily Adams, a Clinton campaign spokeswoman.

“Hillary Clinton was in Iowa on the first day of early vote in person and suggested to all of the attendees of the rally that they go vote,” Adams said. Similarly, President Obama held a rally for Clinton in Ohio just before early voting began in that state.

“The DNC is dominating early voting [outreach]” in Nevada, Ralston said. And it seems to be paying off: So far, the proportion of Nevada early voters who are Democrats is higher than the proportion of registered voters who are Democrats, which suggests Clinton’s lead in the polls there may be mirrored in the results.

Harry Enten contributed reporting.