The United States Constitution stipulates that a presidential election is to be held once every fourth year. The process of electing a president and vice-president, however, begins long before election day.

The nominating process within the political parties officially begins with the first state primaries and caucuses, which usually occur in the month of February of the election year. These primaries and caucuses choose slates of delegates (usually pledged to support particular candidates) to represent the state at the national party conventions.

At the national party conventions, traditionally held in the summer, the delegates from the states cast votes to select the party's candidate for president.

On election day -- the first Tuesday following the first Monday in November of an election year -- every citizen of legal age who has taken the steps necessary in his or her state to meet the voting requirements (such as registering to vote) has an opportunity to vote. However, the president is not formally chosen by direct popular vote. The constitution calls for a process of indirect popular election known as the Electoral College.

• The Presidential Field: Full Coverage of the 2008 Candidates

• Candidates on the Issues

• Candidate Calculator

• U.S. Electoral College

• How do Caucuses Work

• Presidential Elections in the United States: A Primer  (PDF)

• 2008 Presidential Primary Calendar