A brief and somewhat tendentious historical overview of papal elections was given by José Calasanz Vives y Tutó in his Compendium iuris canonici (1905):
In the first ages of the Church, the Clergy and the people of Rome (some say the people were not involved) came together to elect the Pontiff, but, as is clear from history, the right always belonged to the Clergy. Subsequently, i.e. from the year 467, secular powers dared to interfere in the matter, to the extent that, for the sake of peace, a confirmation of the election was expected from the [Holy Roman] Emperor. The Church tolerated this for a certain time in order to prevent greater evils; at length, she recovered her liberty. At any event, from the year 1059 Nicholas II gave the power principally to the Cardinal Bishops, and Alexander III then gave it to all the Cardinals.
The practice of the Pontiff being elected by the Cardinals is entirely fitting because:
1. No-one can better judge the qualities of the man who is elected than the counsellors and assistants of the Roman Pontiff....
2. In this way, schisms more rarely occur....
During vacancies of the Apostolic See, it was the custom for several centuries for the monarchs of Austria, France and Spain to declare to the Cardinals assembled in Conclave that it would be unacceptable to them if a certain Cardinal were elected Pontiff.... This veto was commonly called the exclusiva, although in truth it did not exclude the person concerned. Indeed, the person excluded was often elected. It is therefore a very serious error to defend the exclusiva as if it were a right, or even a mere privilege. It was usurpatory in nature, and was never directly or indirectly approved by the Church.The last attempt to use the exclusiva came from Emperor Franz Josef I of Austria in 1903. It is credited with dashing the prospects of Leo XIII's most obvious successor, Cardinal Rampolla. Leo's actual successor, Pius X, strictly forbade the use of the exclusiva and made the cardinals swear an oath, prior to commencing voting, not to attempt to wield it on behalf of their monarchs.
Until John Paul II issued the Apostolic Constitution Universi Dominici Gregis in 1996, there were three ways by which the cardinals could reach their decision: by acclamation, by scrutiny or by compromise.
Acclamation involved the cardinals as a body hailing the successful candidate as being chosen by the Holy Spirit. It was very rarely used, the last known case being Innocent XI in 1676, but it sometimes comes up in fiction (as in Morris West's The Shoes of the Fisherman and Dan Brown's Angels and Demons).
Scrutiny is the regular process of election. It continues until a candidate has obtained a two-third majority - a rule that dates from 1179. John Paul II ordered that a simple majority would suffice after 34 ballots had been carried out, but this would merely have allowed a majority faction to sit tight and install their candidate by default after the requisite number of ballots had been completed, so the older rule was restored by Benedict XVI. The ballot papers contain the formula "Eligo in summum pontificem" ("I elect as supreme pontiff"). This is now the only permitted method of selecting a pope.
Compromise was an elaborate procedure for use when a conclave was deadlocked. A process of this kind last seems to have been used in the 14th century.
It is well known that any male Catholic can be elected as pope, but the last non-cardinal to be elected was Urban VI in 1378. There have been rumours of non-cardinals receiving a small number of votes in recent conclaves: in 1958, Archbishop Giovanni Montini (later Pope Paul VI) reportedly received a few votes, as allegedly did Archbishop Marcel Lefebvre (the well-known ultra-traditionalist leader) in 1978.
In 1975, Pope Paul VI banned cardinals over 80 from voting. This caused considerable bad feeling at the time.
At the start of the conclave, the participants are sworn to strict secrecy. In practice, however, conclaves leak like sieves. Cardinal Cormac Murphy-O'Connor has written an article for the BBC on what happens in a conclave from the point of view of a participant. Wikipedia has tallies of votes, drawn from various sources, for the 1958, August 1978 and 2005 conclaves. Other tallies are available elsewhere, e.g. in Pontiff by Gordon Thomas and Max Morgan Witts.
• The longest conclave in history ran from 1268 to 1271. The shortest, in 1503, lasted only 10 hours.
• Some liberal Catholics (including the American commentator Fr Andrew Greeley) have suggested that the cardinals' monopoly on electing popes should be broken, and that other bishops, or representatives of priests and laypeople, should be included in the process. These calls go back at least to 1876 and an organisation called the "Società cattolica italiana per la rivendicazione dei diritti spettanti al popolo cristiano ed in specie al popolo romano", which was immediately condemned by the Vatican. In former times, the idea of mass participation in papal elections was met with strong disapproval, particularly after the Protestant Reformation had popularised the idea of lay governance in the church. St Robert Bellarmine, the leading theologian of the papacy, summed up why it was a bad idea (De Clericis, Book 1, Chapter 7):
"First, the common people are inexperienced, and they cannot, even if they greatly desire to, judge whether someone is suitable for priestly office or not.... For how, I ask, would businessmen, farmers, stonemasons and other tradesmen judge whether a person has the learning and prudence necessary in a bishop? Second, if the people had the right of election, the inevitable result would be that the rulers chosen for them would always be those whom the worse and more stupid of them wanted. For the majority would prevail, and more people in every state are bad than good, more stupid than wise.... Finally, popular election is obnoxious because it involves disturbances and subversion."• It is sometimes said that at the conclaves of 1958 and/or 1963 the archconservative Cardinal Giuseppe Siri was initially elected and was then immediately pressured into resigning (Siri was a hardline anticommunist, and his election might have precipitated persecutions of Catholics behind the Iron Curtain). There is some evidence to support this theory, and the FBI apparently accepted it. For example, in 1958 the rituals signalling that a pope had been elected were set in motion part-way through the conclave, only for the cardinals to suddenly go quiet again. Siri himself refused to comment when he was questioned on the subject. We can, however, safely dismiss the conspiracy theory that Siri secretly remained pope (as "Gregory XVII") and founded his own papal line of succession that continues to this day.
• What if all the cardinals suddenly died? In previous ages, this might have resulted from a plague or war. In our own times, it might result from a terrorist attack. A number of theologians have discussed the question. The solutions offered mostly consist of variations on two themes: a new pope could be elected either by a council of all the world's bishops or by the clergy of the diocese of Rome (since the Pope is the Bishop of Rome).
• Benedict XVI was the first pope to resign since Gregory XII relinquished the papacy in 1415. It is widely reported that Pius XII intended to resign if the Nazis, who occupied Rome from 1943 to 1944, violated the Vatican's neutrality and took him prisoner. There were also rumours that John Paul II wrote an undated letter of resignation and placed it in a Vatican safe for use by his aides if he ever became completely incapacitated.