Abstract
The Liber Extra, the most widely circulated collection of medieval canon law, was commissioned by Gregory IX and assembled by the Dominican Raymond of Penyafort. Gregory IX's bull of promulgation, Rex pacificus (1234), ordered that it would be the only collection used in the schools and in the courts and that no other collection was to be made without the special approval of the Apostolic See. It would be Gregory IX's major contribution to the development of medieval canon law. This chapter focuses on the genesis and intentions behind the Liber Extra, the pre-Gregorian sources for the compilation, and Gregory IX's own contribution, before offering suggestions for the path of future scholarship concerning the Liber Extra and Gregory's own legal accomplishments.