What is a caucus?

Caucuses are meetings of registered Democrats and Republicans who gather across the state to show their support for the competing candidates for the party's presidential nomination. But voters can register, or change affiliation, at the caucus - which makes the results unpredictable

How many caucuses are there?

There will be almost 1,800 meetings across Iowa tonight, varying from a handful of people gathering in someone's home, to much larger meetings - and sometimes three or four at the same time - in town halls, schools, churches or other public places

How many people take part?

Only a tiny proportion of the population of a small state. In 2000, the last year in which both parties held caucuses, 59,000 Democrats and 87,000 Republicans voted, in a state with 2.9 million people. In 2004, when the Republicans did not hold a caucus, 124,000 people turned out for the Democratic caucuses

What happens?

Republicans hold a straw poll; Democrats divide into groups favouring particular candidates. On the Democratic side, the supporters of any candidate getting less than 15% support at that caucus are asked to realign themselves to another candidate. The winning candidates' names are sent to a central count, which then announces statewide results

What are the advantages and disadvantages?

It is participatory democracy in action, and many people only decide - or change their minds - at their caucus. But people must attend in person; there is no proxy voting. It takes an entire evening. If the weather is bad, attendance can be low. The system makes it hard for parents, night workers and disabled people to participate. And resentment is growing over the influence of such a small state, which is overwhelmingly white

Why are these caucuses so important?

Tonight's votes are the first test of support, and the winners can expect to get a big boost before next Tuesday's New Hampshire primary. But not all candidates have campaigned in Iowa (Rudy Giuliani has ignored it, preferring to put resources into bigger swing states such as Florida). And the race is so tight that tonight's results may not produce clear winners