The debate’s moderator is well-known TV moderator Morteza Heidari, He also hosted the 2013 debates.
No woman has ever been allowed to run. 137 registered to run in the May vote, all were disqualified, including activist and editor Azam Taleghani.
Read our profile of Taleghani who has already been rejected three times:
The start of the debate has been delayed by 30 minutes. It will now begin at 4:30 p.m. local time, that's 12 p.m. GMT.
Farzan Sabet, a nuclear security fellow at Stanford University and managing editor of a website, IranPolitik.com, has a look at the three main candidates: