A body of resident clergy had been established at the east end of the walled Roman city in the early Middle Ages: the first epigraphic and archaeological evidence of their presence belongs to the 6th century, but the origins were earlier (4th century).  By the 9th century the story of the 3rd-century mission of Saint Firmin had been firmly established.  In the early Middle Ages the cathedral site included multiple buildings: twin churches dedicated to S-Firmin and Notre-Dame, a hospital and baptistery as well as the fortified residence of the bishop in the extreme north-east corner of the Roman wall and houses for the resident clergy to the south.  Systematic organization of the clergy as a group or chapter of canons came in the 9th century. 

In the wave of church building of the twelfth and thirteenth centuries the multiple small churches of the early Middle Ages were often subsumed in a single great church.  Amiens Cathedral had been rebuilt in the mid-12th century but this church was badly damaged by fire in 1218 leading to the construction of the Gothic cathedral.  At that time there were forty canons each of whom received a stipend known as a prebend derived from the agrarian holdings of the chapter as well as rights over the commercial, industrial and jurisdictional life of the city.  Particularly valuable was the status of the chapter as lord of the waters (seigneur des eaux), controlling the multiple waterways that powered the mills employed in the burgeoning textile production industry, as well as the ports used in navigation on the river Somme along which raw wool from England traveled upstream and indigo blue dye (woad) downstream for export.  Canons lived a double life: their families might form part of the urban power elite, yet the daily life of a canon bore some resemblance to that of a monk, requiring them to participate in the Divine Office (based principally upon chanting the Psalms) at fixed (canonical) hours during each day.  This double life was reflected in private ownership of their own houses located in the "cloister" to the south of the cathedral.

In the period of the start of construction on the Gothic cathedral, bishops were elected freely by the chapter: most of them belonged to local seigneurial families; many had already held office in the Amiens chapter.  Their continuing physical presence in the cathedral is marked by their tombs, many of which still survive--you can visit them in the website.  Evrard de Fouilloy and Geoffroy d'Eu were buried at the west end of the choir; Arnoul de la Pierre in the axis of the sanctuary hemicycle and Gerard de Conchy in the outer wall of the choir north aisle. Later in the Middle Ages bishops were often imposed by king or pope: royal patronage promoted Guillaume de Macon (tomb lost) while Jean de la Grange was very close with the papacy: his tomb, originally on the north side of the choir was demolished but the tomb effigy was relocated to the axis of the hemicycle. 

We have very few tombs belonging to deans: the exception is the tomb of Dean Adrien de Hénencourt set in the southern lateral choir screen, which he had paid for.  This dean was a remarkable sponsor of the building projects that mark the end of the life of the Gothic cathedral: the installation of the iron chain to save the crossing piers from collapse, the rebuilding of the flying buttresses, the two rose windows, south and west, the southern choir clôture and the choir stalls and, finally, the steeple.