In the US, the first head-to-head debate between the two leading Presidential candidates takes place tonight.

Hillary Clinton and Donald Trump will face off in a 90-minute debate which is set to be watched by a domestic audience of over 100 million.

Clinton retains a lead over Trump in most average 'polls of polls', but that lead has drawn much narrower in recent weeks.

Analysts believe this is in part because of distrust over Clinton and her perceived secrecy about her health, as well as lingering concerns about her use of private email servers, and about the work of the Clinton Foundation.

Trump has meanwhile grown more popular as he tones down the scale and frequency of his personal attacks, and instead stick to prepared scripts.

That discipline could, however, be tested tonight - while Clinton will also need to avoid the mistakes of previous campaigns, where candidates have come across as cocky and contemptuous.

Previous debates have been a perfect stage for well-timed, scripted gags. But in the modern electoral cycle, equally important is the need to avoid becoming exposed to damaging parodies or satire afterwards.

Our political correspondent Gavan Reilly has been looking at the challenges each candidate will face tonight: