A primary election (nominating primary), also referred to simply as a primary, is an election in which voters in a jurisdiction select candidates for a subsequent election. In other words, primary elections are one means by which a political party nominates candidates for the following general election. "Primaries" are common in the United States, where their origins are traced to the progressive movement. There, primary elections are conducted by government on behalf of the parties. Elsewhere in the world, the nomination of candidates is usually the responsibility of the political party organizations themselves and does not involve the general public.

Besides primaries, other ways that parties may select candidates include caucuses, conventions, and nomination meetings. Historically, Canadian political parties chose their candidates in party meetings in each constituency. Canadian party leaders are elected at leadership conventions, although some parties have abandoned this practice in favour of one member, one vote systems.

Types

• Closed. Voters may vote in a party's primary only if they are registered members of that party. Independents cannot participate. Note that due to the use of the word "independent" in the names of some political parties, the term "non-partisan" is often used to refer to those who are not affiliated with a political party.

• Semi-closed. As in closed primaries, registered party members can vote only in their own party's primary. Semi-closed systems, however, allow unaffiliated voters to participate as well. Depending on the state, independents either make their choice of party primary privately, inside the voting booth, or publicly, by registering with any party on Election Day.

• Open. A registered voter may vote in any party primary regardless of his own party affiliation. When voters do not register with a party before the primary, it is called a pick-a-party primary because the voter can select which party's primary he or she wishes to vote in on election day. Because of the open nature of this system, a practice known as "raiding" may occur. "Raiding" consists of voters of one party crossing over and voting in the primary of another party, effectively allowing a party to help choose its opposition's candidate. The theory is that opposing party members vote for the weakest candidate of the opposite party in order to give their own party the advantage in the general election. An example of this can be seen in the 1998 Vermont senatorial primary with the election of Fred Tuttle for the Republican candidate.

• Semi-open. A registered voter must publicly declare which political party's primary that they will vote in before entering the voting booth. When a voter identifies their self to the election officials they must request a party's specific ballot. Only one ballot is cast by the voter. In many states with semi-open primaries, election officials or poll workers from their respective parties record each voter's choice of party and provide access to this information. The primary difference between a semi-open and open primary system is the use of a party-specific ballot. In a semi-open primary a public declaration in front of the election judges is made and a party-specific ballot given to the voter to cast. Certain states that use the open-primary format may print a single ballot and the voter must choose on the ballot itself which political party's candidates they will select for a contested office.