Juan Carlos: Does Spanish monarchy speak for country's youth?
18 June 2014 Last updated at 15:43 BST
After 39 years in power, King Juan Carlos will sign the bill of his abdication, and his son Felipe will be proclaimed King at a ceremony in Madrid.
The transition has sparked a debate about the possibility of holding a referendum in Spain to decide on the future of the monarchy.
The BBC's Chris Morris spoke to two young Spaniards at Madrid's Plaza Mayor about that debate.