The chapel also functions as the cathedral treasury (Tesoro) and is elaborately decorated with Baroque art. There are no less than seven altars, four of which have paintings by Domenichino. A tabernacle behind the main altar enshrines a silver reliquary bust (1305) containing the head of St. Januarius. A large gilt-bronze grille separates the chapel from the nave.

The cathedral is dedicated to San Gennaro (St. Januarius), Bishop of Naples, a martyr saint of both the Roman Catholic and the Eastern Orthodox Churches (died ca. 305) and the patron saint of Naples. It was built on the foundations of two pre-Christian basilicas, and completed in the early 14th century under Robert of Anjou. With the transfer of the remains of San Gennaro to the cathedral in 1497, a new chapel to house the relics was commissioned. The Succorpo (crypt) was completed in 1506 after a design by Tommaso Malvito of Como. The chapel, its attendant sculpture and its wall decoration mark a high-point of Renaissance style in Naples. The west facade was renovated in a Neo-Gothic style in 1877-1905 by Enrico Alvino; only the portal survives from the medieval facade.