The race is on for Americans to choose the next president of the United States.
The contest for who will face off against the controversial current President Donald Trump started with more than 20 candidates but the list will continue to narrow down as primary season kicks off.
Although third parties can have candidates, the main race is between the left-wing Democratic Party and the right-wing Republican Party.
The parties elect their candidates through primaries or caucuses in each of the states and US territories.
Donald Trump is widely expected to be the Republican candidate despite being embroiled in an impeachment process. So all eyes will be on the Democratic party as the large field of competitors is reduced to a single presidential candidate.
Here's a look at how the parties nominate their candidates before the presidential election on November 3.
Iowa is the first state to vote for party candidates and they do it through what's called a caucus, an important early step in the primary election season.
A caucus is a gathering of voters at a specific time of the day where party members pick their candidate. The meetings occur in churches, schools, town halls and other local venues and are funded by the political parties.
Democrats gather together in groups and cast votes publicly. The voters can even change candidates by walking to join other voters if their chosen candidate does not get 15% of the vote. Republicans also attend local meetings but cast a ballot at those meetings, experts say.
"The influence is less about the number of voters, than the very fact that it is first. But it is also about how engaged Iowa voters are, and how they go to events, see candidates, and often challenge them with questions," said University of Delaware professor David Redlawsk.
Redlawsk said that the Iowa Caucus is determined in many ways by expectations. A candidate that does better or worse than expected could influence their future prospects.
Candidates are aiming to win these states by winning delegates - who are assigned locally to essentially represent the voters and who will nominate the candidate at the later convention.
But most states hold a primary election, funded by the state government. This is where voters go to a polling place (normally open all day) to cast a secret ballot, Redlawsk said.
The primary can be open or closed, meaning that in certain states, voters that do not belong to a specific party can vote in that party's primary.
After the Iowa Caucus, there is a primary election in New Hampshire, a caucus in Nevada and a primary election in South Carolina.
After the early states narrow down the Democratic field, several states will hold primary elections all at once - on a day called Super Tuesday.
Elections in the US are often held on Tuesdays, but on March 3, there are 14 states that will hold primary elections.
This will be a big moment for the Democratic party, especially since the two most populous states, California and Texas, will hold their primaries on Super Tuesday this year, Redlawsk said.
"About 40% of all delegates will be elected for the Democrats on Super Tuesday, which means a lot will be at stake," said Redlawsk.
There are still several states that will hold primary elections through June. The Associated Press reported that Hillary Clinton would be the democratic candidate in early June 2016. Trump was reported to have enough delegates to become the Republican nominee in May 2016.
Parties officially nominate their candidates in a convention - a large gathering in which the candidate gives a speech and the delegates officially cast their votes for the candidates.
This year the Democrats will hold their convention in July and the Republicans hold their conventions in April. This marks the end of the primary season as the national campaign begins for the official candidates of each party.
The election will take place on November 3, 2020, where voters cast their ballot in their home state. Each state has a select number of electors.
Presidential candidates have to win at least 270 of the 538 total electors in order to win the race.