Ted Cruz has caused a big upset by beating Donald Trump in the Republican Iowa caucuses, while Hillary Clinton claimed an extremely narrow victory over rival Bernie Sanders in the Democrats' race.
The caucus is the first vote in the race for the US presidential nominations for both parties, and is seen as an important bellwether for candidates' chances in the elections.
Bernie Sanders says he and Hillary Clinton are in a "virtual tie" to win the Democratic Iowa caucuses.
Sanders said he expected to have half the Iowa delegates campaigning for him at the Democratic National Convention in July, where the party's candidate will be confirmed.
The self-style Democratic socialist said "given the enormous crisis...it is too late for establishment politics and establishment economics."
Bernie Sanders' brother says he thinks his sibling has what it takes to win the next US Presidency.
There is much still to play for in both parties in the first real test for America's would-be presidents.
Ted Cruz caused a huge upset by beating Donald Trump in Iowa's Republican caucuses.