[1][2] When it was the archbishopric and Electorate of Trier, it was one of the most important states of the Holy Roman Empire, both as an ecclesiastical principality and as a diocese of the church.

Unlike the other Rhenish dioceses—including Mainz and Cologne–Trier was the former Roman provincial capital of Augusta Treverorum.

Given its status, Trier has continuously been an episcopal see since Roman times and is one of the oldest dioceses in all of Germany.

[3] The bishops of Trier were already virtually independent territorial magnates in Merovingian times.

From 1795, the territories of the Archbishopric on the left bank of the Rhine — which is to say almost all of them — were under French occupation, and were annexed in 1801 and a separate bishopric established (later assuming control of the whole diocese in 1803).