Liber Pontificalis is a major source for the history of the papacy in the first millenium. It is a collection of the lives of popes, starting from St Peter and kept going through to 870. Liber Pontificalis is prefaced by two apocryphical letters of Pope Damasus and Jerome, but it cannot be dated to that period. Although Mommsen tended to put the date of the actual compilation as late as the seventh century, nowadays Duchesne`s view is generally accepted that there were two editions made in the 530s-540s. The first, presumably completed soon after 530, has not survived as such, though we have two epitomes made from it (known as “Felician” and “Cononian” from the names of the popes at which they end). Duchesne tried to reconstruct it in his edition, but we follow the second edition presented by him, which was completed by the siege of Rome in 546. The work was then left aside for some time, and taken up again probably under Honorius (625-638) or shortly afterwards; hence the additions were written shortly after each pontiff`s death.

Liber starts to provide some more reliable information with the times of Pope Leo I (440-461), and becomes very well informed with the end of the fifth century. The lives of earlier popes cannot be considered as a valid source of information about their lifetime. However, those notices are a precious source for the sixth century: we learn what was considered an old tradition at the time, and how the past of the Roman church was being seen and constructed then. It is especially important when we deal with the liturgy.