The basilica of Ayios Demetrios possesses a strong emotional appeal to the people of Thessalonike, while at the same time it constitutes a museum of Byzantine art on account of the successive phases of its building (5th and 7th centuries, and reconstruction in the 20th century). It was erected over the site of the Roman baths where the saint was martyred and where the earliest place of his worship once stood (4th century).
Its form, a five-aisled basilica with an additional transverse aisle in front of the sanctuary, resembles a cross in plan, denoting the site as a place of martyrdom. The sculptured architectural members and marble panelling of the walls vest the monument with the magnificence that befits the martyr Demetrios.
The decoration is complemented with 7th century wall-paintings and mosaics dating from the 5th to the 7th centuries and to the 9th century; they lack iconographic coherence, the main subject being the saint portrayed with the donors of the church or with a variety of other figures.
The mosaics are unique and of particular note in connection with the art of the Early Christian period, for in this instance the principles of Hellenistic art have not been entirely abandoned, although painting is becoming gradually more flat and more abstract.