"Ad limina Apostolorum" is an official visit and prayer on the graves of apostles St. Peter and Paul in Rome.
Every bishop is obligated to make this pilgrimage. The ad limina is considered an important moment in the exercise of the Holy Father"s pastoral ministry.
Pope Sixtus V (1585-1590) made the ad limina normative and imposed (apparently for the first time) the obligation of informing the pope periodically about the material and spiritual state of local churches (dioceses).
The remains of St. Peter lay under the main altar of St. Peter"s Basilica, while the Basilica of St. Paul Outside the Walls marks the burial place of St. Paul.