Church councils formed an integral part of medieval canon law, indeed one might say its backbone. Nicaea I in 325 will be our starting point, for, although this council belongs to the early Church, its influence upon medieval canon law was profound. It was first in the list of councils that came to be regarded as ecumenical, that is, as a council of the whole Church. Although ecumenical councils form the main focus of this chapter, attention will also be given to local councils, which played important roles in canonical development.

Decretals, epistolae decretales, are papal letters that have a claim to universal validity and clarify questions of Church law. Already in Late Antiquity, petitioners would submit legal or disciplinary questions to the Roman emperor, who in response would provide authoritative answers in imperial rescripts. Papal decretal law was the product of an analogous procedure. Private parties would ask the pope to adjudicate their disputes, and in response the pope would set forth authoritative answers in decretal letters. The oldest fully preserved papal decretal is by Pope Siricius (384–99). Only with the pontificate of Alexander III (1159–81), however, did the number of decretals skyrocket and, as a result, there take place the further legal development and elaboration of the ius novum. Just two generations after Gratian’s Decretum (c. 1140), papal legislative acts had developed and changed canon law like never before.