By the side of the Civil Law, there grew up a second system of laws, known as the Canon Law. The development of the Canon Law dates from about the fourth century, and was the work of the Western Church. The sources of this law were the canons of the church synods, the decretals of the popes, supplemented by the incorporation of many Roman law principles. A number of codes or compilations of canons and decretals were made, beginning near the close of the fifth century, but the first satisfactory treatise on the whole subject was only completed about the year 1150 by Gratian, a monk and a professor of Canon law at the University of Bologna. This work was generally known either as the Decretum or the Corpus Juris Canonici.

The work of Gratian soon became antiquated, and a number of new works on the subject followed each other in rapid succession. The first great official collection was published in 1234, under the authority of Pope Gregory IX. A second official collection of church laws was made under Pope Boniface VIII, and a third under Clement V. These three collections, together with the Decretum Gratiani, became the basis of Canon Law.

The Canon Law early became a distinct system of jurisprudence, with ecclesiastical courts for its enforcement throughout all western Europe.