In the Roman Republic, the Pontifex Maximus was the head of the Roman religion. He was the most important of the Pontifices (plural of Pontifex), which were positions in the main sacred college (Collegium Pontificum), which was directed by the Pontifex Maximus. Other members of this priesthood included the Rex Sacrorum (king of the sacred rites), the Flaminii, and the Vestales. The number of Pontifices, elected by cooptatio, was originally six, but this number increased to fifteen in the 1st century BC. The Pontifices served for life. The office came into its own with the abolition of the monarchy, when the sacral powers previously vested in the king were transferred either to the Pontifex Maximus or to the Rex Sacrorum. Today it may still be said that the head of the Roman religion is the Pontifex Maximus, since that is one of the titles claimed by the Pope.

The Pontifex was not simply a priest. He had both political and religious authority. It is not clear which of the two came first or had the most importance.

The Pontifices had many relevant and prestigious functions, such as keeping the official minutes of elected magistrates (see Fasti), and the so-called "public diaries", the Annales maximi. They also collected information related to the Roman religious tradition into a sort of corpus which summarised dogma and other concepts, similar to later compilations of law in Jurisprudence.

Some authors believe that eventually Roman magistrates would have gained some of the Pontifices' political prerogatives and powers. Earlier Pontifices were elected only from the noble class, but in 300 BC the lex Ogulnia admitted people from plebs too to run for the charge, so that part of the prestige of the title was lost.

In 104 BC the lex Domitia prescribed that the election would henceforward be voted by the comitia tributa; by the same law, only 17 of the 35 tribes of the town could vote. This law was abolished by Sulla and restored when Julius Caesar was a Pontifex Maximus. A further modification prescribed then that the Emperor would have statutorily been the Pontifex Maximus and would have personally named the other Pontifices. With this attribution, the Emperor was given a religious dignity, completing the greatness of the throne. Most authors contend that the power of naming the Pontifices was not really used as an instrumentum regni, an enforcing power.

Tertullian first applied the term to Pope Callixtus I, although Pope Gregory I was the first to employ it in any formal sense. Pontifex was apparently a word in common currency in early Christianity to denote a bishop. The office was relinquished by the Emperor Gratianus in 382, and was assumed by the Christian Bishops of Rome. It thus became one of the titles of the Popes of the Roman Catholic Church who hold it to this day. This is unusual in that most of the technical terms of Roman paganism were avoided in the vocabulary of Christian Latin in favour of neologisms or Greek words.

In Latin, Pontifex comes from pontem faciens, and means "bridge-maker". This was indeed an important position in Rome, where the major bridges were over the Tiber, the holy river (and a deity, at the same time); only prestigious authorities, with sacral function could be allowed to "disturb" it with mechanical additions. Other experts believe that the position, in its religious interpretation, would have provided men with a symbolic "bridge" to let them contact the gods; it has besides been noted that in ancient India similar concepts were in use in similar ages, here too ideally regarding rivers and bridges. The word has also been thought by some to be a corruption of a similar-sounding but etymologically unrelated Etruscan word for priest, but this theory is a minority opinion.

Incomplete list of Pontifices maximi