Pope Benedict XVI’s resignation at the end of this month citing health concerns has taken the world by surprise, thrusting the Catholic Church into a realm it hasn’t entered in more than 600 years.

Not since Pope Gregory XII resigned from his position to end the Western Schism in 1415 has there been a case for a Pope to willingly leave. Pope Benedict XVI joins a handful of others who have resigned from the position, whether it be bribery (Pope Gregory VI), marriage (Pope Benedict IX) or simply because it was just too hard (Pope Celestine V).

But, Pope Benedict XVI’s announcement has set the stage for the introduction of the Church’s third Pope of the 21st Century.

So how is a Pope selected? In the usual case (i.e. When the Pope dies), there are nine days of mourning, followed by the Conclave of Cardinals. The Conclave is held no sooner than 15 days and no later than 20 days after a Pope’s passing, with 120 members- forming the College of Cardinals- making the decision. From there, a special voting ceremony is held four times a day until a new Pope is selected.

Voting is a secretive process using ballots checked by three Scrutineers, who validate each vote and crunch the numbers. To be selected, a candidate must receive a two thirds plus one vote. Unsuccessful elections are denoted by black smoke, created by either putting water or other chemicals on the burning ballots. After 12 or 13 days, if no one is selected the Conclave may opt to elect a Pope by simple majority.

White smoke is the result of a successful vote, in which case a new Pope is elected. Soon after his selection, the newly designated Pope may choose his own name. IN John Paul II's case, he chose his name based on his predecessor, John Paul I, who died after only 33 days in power.

The past two conclaves have ended rather quickly, as current Pope Benedict XVI was elected in four ballots. His predecessor, John Paul II, was selected in eight ballots during the course of two days.

As for who is in the running to be the next Pope, no one quite knows yet. Bookies on the Irish betting site Paddy Power are already starting to lay out the odds, with Canada’s Cardinal Marc Ouellet leading the pack at 5/2. Other notables include Nigeria’s Francis Arinze (6/1) and Ghana’s Peter Turkson (4/1). The highest American cardinals sit a little lower on the chart, as Cardinals Raymond Burke and Timothy Dolan are each granted 25/1 odds of ascending to the pinnacle of the Catholic Church.

In any case, the world’s Catholics will be holding their collective breath as they wait for the white smoke to billow from the Vatican.