The Diocesan Museum collection has over 600 works of art, of which almost 400 are on display, divided into various sections.
The Quadreria Arcivescovile picture gallery has contributed many of the artworks in the Museum, from the collections created by Milan's archbishops, providing an overview of the cultural preferences of those who succeeded Ambrose on the archbishop's throne. For instance, the Museum has part of the Visconti and Pozzobonelli collections, and the entire Erba Odescalchi collection.
The Diocesan Museum also contains numerous works from the Diocese, dated sixth to nineteenth century. Following a meticulous study of the Diocese's Artistic Heritage Archive, moved to the Museum in 1998, prior to its inauguration investigation and visits to the various parishes enabled the identification of some real hidden treasures. This was the start of the recovery and valorization of the Diocese's artistic heritage, which is one of the Museum's chief aims. This nucleus of works was enlarged by the addition of a section dedicated to St Ambrose, the Fondi Oro (artworks made on a gold background, produced mainly in Tuscany during the fourteenth and fifteenth centuries, collected by Prof. Alberto Crespi, and donated to the Museum in 2000), sculptures and paintings from the Marcenaro collection (loaned by the Fondazione Cariplo in 2004), the cycle of canvases from the Arciconfraternita del Santissimo Sacramento and, lastly, a significant section dedicated to religious furnishings. In the near future there are plans to dedicate an entire section to the 1900s. The Museum already has work by twentieth-century artists (Lucio Fontana, Aldo Carpi, William Congdon and others), but these are mainly in storage at the moment.