The US presidential and legislative elections will be held on November 6, 2012. But they will be preceded by a months-long process -- starting with Iowa's caucus on Tuesday -- during which Republicans will pick a candidate to take on Democratic President Barack Obama.
Here is a guide to caucuses and primaries -- a system virtually unique in the world. Only Republicans will be picking a candidate this time, with Obama expected to be nominated unchallenged as the Democratic Party's candidate.
-- CAUCUS: used in about a dozen states when in simple gatherings supporters from each party usually meet to decide their delegates to the national party convention and who they will vote for on the party's final ticket.
Voters gather at schools, fire stations, and other public buildings, and divide themselves into groups around each candidate. Those who are undecided are then "courted" by the other groups.
At the end of the process, party officials count the number of voters in each candidate's group and then decide how many delegates to the convention each has won.
In 2008, Obama won over his Democratic rival Hillary Clinton in Iowa. Mike Huckabee took the honor for the Republicans, but was later defeated for the Republican nomination by veteran Senator John McCain.
The Iowa caucus sets the whole election calendar in motion, and will be held on January 3. It acquired its importance in the 1970s when the contest began to draw an army of television cameras, but it matters less for who is chosen and more as a way of gauging support for the various candidates.
-- PRIMARY: Primary elections feature secret ballot voting, similar to that on election day. Most states hold primaries to select their delegates to the party convention and which candidate they should back.
New Hampshire generally holds the nation's first primary, and it is expected to be held on January 10, 2012. It is an asset that gives the tiny northeastern state vast influence on the overall electoral process.
A primary may be open or closed. In the first instance, voting is not restricted by party membership, so registered Democratic voters could vote in the Republican primary, while in the second, only registered party members may cast ballots.
Florida this time has squeezed the election timetable by announcing it will hold its primary on January 31, meaning a Republican frontrunner could emerge early next year.
Super Tuesday on March 6 could prove a decisive day when primaries are due to be organized in 10 states. The final primaries are due to held in June 2012.