The cathedral treasury houses precious reliquaries, liturgical equipment, manuscripts, vestments and insignia of the archbishops and cathedral clergy from the 4th to the 20th century, some of which are still used in today’s practices. The treasury, whose beginnings can be traced back to the 9th century, is presented on the north side of Cologne Cathedral in the converted historical cellar vaults of the 13th century that lead down to the Roman city wall and a Roman sewer tunnel.

The remarkable collection of artisanal works features a precious 18th century bishop's vestment, the Capella Clementina. It was commissioned in France in 1742 by the Cologne Elector and Archbishop Clemens August for the coronation of his brother Emperor Karl VII. Today the collection comprises 44 pieces, including 22 gold-embroidered vestments. It also includes examples of excellent German craftsmanship such as the Apocalyptic Lamb, a work by the Cologne goldsmith Johann Heinrich Rohr from 1775.