En español | Voting in the nation's presidential primaries and caucuses begins Feb. 3 and runs through June 2.
The rules can be complicated, so it pays to know the voting requirements before you head to the polls. Are you eligible to cast a primary ballot? Do you need to register in advance? What kind of identification, if any, is required? You'll find the answers here.
But first, note that the biggest change in the presidential preference contests this year from 2016 is that fewer and fewer states are caucusing. Unlike a conventional primary, in which each voter simply casts a ballot, a caucus is a simultaneous gathering of voters who meet to select their preference.
This year both parties will hold caucuses only in Iowa and Wyoming. And just Democrats will caucus in Nevada and North Dakota; Republicans will hold a primary. In Hawaii and Kentucky, Republicans will caucus, and Democrats will hold a primary. Since 2016, 11 states have moved from caucusing to holding primaries. Find the date of your state's primary or caucus.
Can I vote in a primary?
The rules that determine who can vote in primaries vary widely by state. Most primaries fall under one of these systems.
Closed: Only voters registered with the party holding the primary can vote. The deadline for affiliating with a party varies by state.
Partially closed: Political parties can decide before each election whether voters not registered with any party (often called unaffiliated voters) may participate in their nominating contest. Under this system, Democrats could allow independents to vote while still excluding Republicans.
Open: Voters may cast a ballot in either party's primary without registering with that party or publicly declaring which ballot they want.
Partially open: Under this system, states allow voters to cast a ballot with either party — but they must ask for a party's ballot publicly. Some states allow unaffiliated voters to vote in either primary but don't let members of one party cross over and vote in the opposing party's contest.
Other: A few states have set up more unusual primary systems. In California, Washington and Louisiana, there's one primary ballot and the top two winners, regardless of party, go on to the general election. In California that often leads to two Democrats facing off in the general election. Nebraska also uses this system but only for its state elections.
To find details of the rules in your state, contact your local or state election office. Find contact information at usa.gov/election-office.
How do I register to vote?
Registration deadlines and rules vary by state. In 37 states and the District of Columbia, you can register online. In 21 states and D.C., you can register to vote and cast your ballot on the same day; if you wish to do this, be prepared to present proof of where you live (such as a driver's license or ID card). In some states you may use a paycheck or utility bill with your address on it as proof of residency. Certain states will also require same-day registrants to sign an affidavit or swear an oath that they are eligible to vote and haven't already voted.
The “motor voter” law requires states to let people register at motor vehicle offices. According to the National Conference of State Legislatures, each year about a third of all voter-registration applications are made via a motor vehicle office.
Find out the registration deadlines in your state by going to the U.S. Election Assistance Commission's website.
Do I need to vote in person?
The most common way to cast a ballot is to go to your local polling place on Election Day, but most states provide an alternate method for those who are not going to be in town that day or are unable to get to the polls. The rules for early voting, absentee ballots and all-mail voting differ from state to state.
The availability of early voting has been the subject of a number of court cases. In Texas, for example, a lawsuit is challenging the state's decision to eliminate some early-voting sites, especially on college campuses. You can find the most up-to-date information on voting rules in your state on the Can I Vote page on the National Association of Secretaries of State website.
Here are the ways you can vote without going to the polls.
Absentee voting: All states will mail you an absentee ballot if you apply for one. Applications are often available online. Some states require you to say why you can't vote in person, but others offer a “no excuse” absentee ballot. Accepted excuses usually include disability or travel.
Early voting: Thirty-nine states and the District of Columbia offer early voting. Some states begin early voting 45 days before the polls open; others provide only a few extra days. And certain states offer voting on weekends. In November, Virginians will, for the first time, be able to vote before the election — without having to provide an excuse.
All-mail voting: In all-mail elections, ballots are sent to all registered voters, who can return them by mail or bring them to a voting center. Oregon, Washington and Colorado historically have had all-mail systems. Beginning with this year's primary, Hawaii will conduct all-mail voting. In 2016, California passed the Voter's Choice Act, which allows counties to conduct all-mail elections. This year 15 of the state's 58 counties have moved to this system.
What must I bring to the polls?
Thirty-four states require voters to show some form of identification on Election Day. The other 16 and the District of Columbia tend to verify identity by, say, asking voters to sign a card and checking the signature against their voter registration card.
Voter ID laws vary by state. Some require a photo ID, such as a driver's license or passport; others accept non-photo IDs, such as a bank statement or utility bill.
Most states have a procedure to enable people without acceptable identification to vote. Some just ask them to sign an affidavit affirming who they are; others let a voter cast a provisional ballot, but the individual then must bring an acceptable ID to the election office within days of the contest to have the ballot counted.