List of 10 longest-reigning popes

The list of 10 longest-reigning Popes in Catholic reckoning, with one extra listing:
St. Peter (30 to AD 64/67): 34 or 37 years (see notes on St. Peter below)
Bl. Pius IX (1846–1878): 31 years and 7 months (11,560 days).
John Paul II (1978–present): 26 years and 142 days or 9 635 days as of 7 March 2005 – he would pass Pius IX on 9 June 2010 if still reigning as Pope; he would be 90 years old on 18 May, 2010.
Leo XIII (1878–1903): 25 years, 4 months and 29 days (9,280 days).
Pius VI (1775–1799): 24 years and 6 months (8,962 days).
Adrian I (772–795): 23 years and 10 months (8,728 days).
Pius VII (1800–1823): 23 years and 5 months (8,559 days).
Alexander III (1159–1181): 21 years, 11 months, and 2 days (8,001 days).
St. Silvester I (314–335): 21 years and 11 months.
St. Leo I (440–461): 21 years and 1 month.
Urban VIII (1623–1644): 20 years, 11 months, 23 days (7,663 days).

Notes on St. Peter
St. Peter's place in the list is a matter of some dispute.
The length of St. Peter's reign is given by traditional sources, but their accuracy is uncertain. (In particular, two different death years are proposed.) Traditionally, St. Peter spent 25 years in Rome, but his term is reckoned from the time that Catholics consider Jesus to have given St. Peter his office.
Some non-Catholics dispute St. Peter being in this list (and the list of popes) at all, on the grounds that the papacy as we know it now did not exist until some centuries after Jesus' death. They would therefore consider the occupants of slots 2–11 in the list above to be the ten longest-reigning popes.
Catholics, on the other hand, consider St. Peter to necessarily be the first pope by virtue of his commission by Jesus and especially for being the first Bishop of Rome, regardless of whether he was generally known by, or personally claimed, that title. By Catholic understanding, all later popes reign by virtue of their succession to St. Peter in his office.