Presidential primaries are party elections and caucuses to determine who the presidential nominee will be for each political party. They are held between January and June before the general election in November.

All party members get to vote in the presidential primaries whereas they do not in the caucus system.

In the US, the presidential and vice-presidential candidates for a party are announced at the party's respective convention. Both the Democratic and the Republican ones occured in fall of 2008.

Five parties nominated candidates for the 2008 U.S. Presidential election, the Republican Party, the Democratic Party, the Libertarian Party, the Green Party and the Constitution Party. In addition to their candidates, Ralph Nader ran as an Independent.