More than 20 US states go to the polls today, yet "Super Tuesday" will not have all the answers.
Presidential hopefuls looking to win their party's nomination first need to win the majority of their party's delegates, who will officially choose the nominees this summer.
These delegates are pledged to support a particular candidate at this summer's conventions, which will be held in Denver on August 25th to 8th for the Democrats and in Minnesota on September 1st to 4th for the Republicans.
For the Democrats, the magic number is 2,025, and 1,681 are available on Super Tuesday.
Hillary Clinton leads the way at the moment with 232, followed by Barack Obama with 158. These figures include CNN estimates for the number of unpledged superdelegates, based on non-binding votes for candidate preferences.
For the Republicans, it is 1,191, with 1,023 available on Super Tuesday. John McCain leads the way with 97, followed by Mitt Romney with 92.
Party policy states that Democratic primary elections must use a proportional system of selecting delegates, with candidates receiving a number of delegates relative to his or her percentage of the vote.
The Democratic National Committee also assigns each state a number of so-called superdelegates. These unpledged delegates are not bound by the state's primary or caucus results and include all members of the Democratic National Committee as well as elected officials such as members of Congress and governors.
They were created after the 1980 presidential election in a bid to ensure party officials were given a substantial voice in the nominating process. Since 1984, they have made up 15 to 20 per cent of the Democratic delegation and have tended to support the front-runner at the convention.
The Republican party allows states to choose either a proportional system or a winner-takes-all system. As a result, the Republicans are more likely to emerge from Super Tuesday with an undisputed front-runner as several states - including New York, New Jersey, Missouri and Arizona - award all their Republican delegates to the candidate who wins the popular vote.
The United States votes on November 4th - but not officially for who will be the country's president come January 20th, 2009.
Instead, they vote for state electors who, in turn, cast their official votes for president 41 days later in a system known as the electoral college, enshrined in article two of the US constitution.
The number of electors each state has is equal to its number of senators and congressional representatives - with the exception of the District of Columbia, which gets three.
The electoral college voting system is similar to that of the primary elections, with some states using proportional representation and others using a winner-takes-all method.
A total of 270 electoral college votes are needed to secure the presidency, but if that does not happen the winner will be decided by the courts.