The “Book of Popes” contains biographies of popes from the Apostle Peter's day to the mid- fifteenth century. The entries give such basic information as each man's birthplace, date of pontificate, and contributions to Catholicism. They tend to increase considerably in size with later pontiffs and are of uneven reliability, often reflecting the biases of the era in which composed. The first edition, based on a fourth-century list of Roman bishops, Catalogus Liberianus, appeared in the sixth or perhaps seventh century. Thereafter Liber pontificalis was periodically updated. It is least adequate on tenth- and eleventh- century popes. Yet it remains a basic source of information and is indispensable as a reflection of medieval attitudes towards the papacy.