The mosaic decoration of the entire upper part of the architecture of St. Mark’s – an area of around 8.000 square metres – is fruit of one unifying idea.
Scholars agree that the grand iconographical plan of the interior was already completed in the course of the 12th century. The mosaics in the interior recount the events of the New Testament, with the great message of Christian salvation.
The mosaics in the atrium, carried out afterwards, during the 13th century, are a meditation on the Old Testament, in particular the books of Genesis and Exodus, and are well located as precursor of and preparation for the interior.
Interwoven with this main plan one identifies many others: the story of the Virgin, the martyrdoms of St. Peter and St. Clement, the events of St. John the Evangelist’s life and those of John the Baptist and St. Isadore, the great pantheon of saints worshipped by the Venetians and, most important of all, the cycles with thelegend of St. Mark.
The gold background of the mosaics does not only give unity to the mosaics themselves but, in accordance with the oriental conception, has a precise symbolic value as the colour of the Divine, the image of that light which, for the theologians and Fathers of the mediaeval church, was God himself.

The mosaic complex of St. Mark’s turns on certain iconographic themes.
Though different interpretations have been made, we are dealing with a theological scheme that distributed the mosaics both outside and inside the whole building. The most accredited hypotheses point to a theologian active in St. Mark’s, perhaps Jacobo Venetico, a Greek and scholar of Aristotle.
However it must be admitted that there are other subsequent iconographies: one for the atrium, another for the external stories of St. Mark, a third for the baptistery, a fourth for the chapel of St. Isadore, a fifth for the Mascoli chapel and a sixth for the sacristy complex.

The mosaic decoration of St. Mark’s covers an area of more than 8000 square metres, chiefly illustrating biblical themes from the Old Testament in the atrium and the New Testament inside the Church.
The events recounted in the Pentateuch (the name given to the first five books of the Bible and attributed to Moses) are set out in the atrium. The first event, depicted in the cupola, is the creation of the world (the hexaemeron) and the story of Adam and Eve. It is one of the masterpieces of world art in the portrayal of divine works in the 6 days of creation. There follow the stories of Cain and Abel, Noah, the Flood, the Tower of Babel, then Abraham and the stories of Joseph, occupying three small cupolas of the northern arm, and ending with the main events of Moses’ life up to the crossing of the Red Sea.
This cycle of mosaics was begun in the early decades of the 13th century, maybe in 1230, and completed in 1275. The series is inspired almost to the letter by the miniatures of the “Cotton Bible“, which dates to perhaps the 5th century and of which there are some fragments in the British Museum. It is a biblical text of late-antique Egyptian (more precisely Alexandrian) origin.

The episodes from the New Testament inside the church centre around the events of the life of Christ as told in the Gospels, the Acts of the Apostles and the Book of the Apocalypse.
The mosaics dealing with the life of the Virgin, situated at the ends of the transept, of Byzantine influence and partly inspired by the apocryphal early Gospel of St. James, should be seen as having the function of linking the two Testaments.
However, those of the early 15th century Mascoli chapel, which are of an exclusively Marian nature, may be considered as a cycle in themselves, referable to specific forms of worshipping the Virgin.

There is also a vast series of hagiographic mosaics from the lives of the saints, in particular regarding:
– St. Mark the evangelist in the two chancels, the west wall of the south transept, the vault of the Zen chapel and on the facade;
– the apostles, on the two vast left and right tribunes at the side of the Pentecost cupola;
– St. Isadore in the mosaics of the chapel of the same name, a saint associated, after the crusades, with military and political luck;
– St. Leonard, the popular saint of Provence, with a chapel dedicated to him where the main events of his life are depicted. His noble aspects however are highlighted: the chapel came under the area of the church that was strictly for the doge’s use.
– the stories of St. Peter and St. Clement, pope, on the lower side of the left and right tribune, at the side of the presbytery. St. Clement’s presence
perhaps refers to the importance of his cult which already in Alexandria was linked to the cult of St. Mark to whom seafaring people were devoted.

In the atrium as in the interior, but here more highly evidenced, the mosaics may also be read along linear vertical progressions from top to bottom.
Usually the upper part deals with episodes from the New Testament and the middle part with isolated figures of prophets who have an interpretative role with regard to the former in accordance with the normal laws of mediaeval criticism which saw in the New Testament the verification of what had been announced on the Old Testament by the prophets and by their lives and words.
The lower register of the fascia concerns the indigenous and patron saints of the local pantheon, in accordance with Byzantine custom, with the addition of foreign saints with whom there were links of piety or who were venerated in countries with which the Republic had trade relationships.