Only cardinals under 80 are allowed to vote in the papal conclave, according to Catholic Church regulations. The historic election process will see participation of Indian cardinals. Around 138 of the 252 cardinals worldwide are currently eligible for the process, according to the College of Cardinals Report website.

Concerns over Pope Francis' health have mounted in recent years as have questions about his succession. Here's what we know.

As Pope Francis remains in critical condition at Gemelli Hospital in Rome - still in charge of the Vatican and the worldwide Roman Catholic church - millions of the devout are praying for his full recovery,

Like his predecessors, Pope Francis was selected using an age-old method -- a system known as the Papal conclave.