Gratian lived some time around the middle of the twelfth century, but very little is known about him as a person. Many textbooks (still) claim that he was a Camaldulese monk and taught in a specific monastery in Bologna, but this has proven to be a historiographical myth.

The only thing we know for certain is that he wrote (or rather compiled) a very important canonical collection, which he called the Concordia discordantium canonum ("The Concord of Discordant Canons"), although it is usually known as the Decretum. The most recent material included in the work is from the second Lateran Council of 1139, so it is usually dated to "circa 1140." The Decretum was very successful and was considered a fundamental law book in the Catholic Church until 1917. There are hundreds of medieval manuscripts of the work.

The Decretum contains almost 4,000 canons, i.e. snippets of legislation. These canons are accompanied by Gratian's commentary, these so-called dicta. Recently, an earlier version of the work has been discovered. This version, called the first recension, is about half as long as the Decretum previously known.

Gratian's purpose with the work is evident already from the title he gave it. The normative legal statements which tradition had handed down to his time, the "canons," often disagreed with each other, hence leaving an ecclesiastical judge in the dark about how to judge a case. As the first canonist ever, Gratian attempted to reconcile the contradictions for the entire scope of medieval ecclesiastical legislation. Incidentally, the Decretum was one of the very first (if not the first) such work in any discipline and is thence an important milestone in the developments of early scholastic method.