In Iowa's caucuses, voters meet in private homes, schools and other public buildings in more than 1,780 districts, or precincts, across the state to discuss the candidates and the issues.
They then elect delegates to the county conventions. County convention delegates elect delegates in turn to state conventions, where delegates to the national conventions are chosen.
At Iowa's Democratic caucuses, the voters publicly divide into groups, gathering in different corners of a room to show their support for the different candidates, and delegates are allocated accordingly.
Voters at the state's Republican caucuses take part in a secret ballot, the results of which inform the allocation of delegates.
Other states' caucus procedures may vary according to state law.
Primary elections, such as take place in New Hampshire, allow all registered voters in a state directly to vote for their preferred candidate.