When the state of Byzantium was established the Aegean ceased to be under Roman domination and came under the rule of the Byzantines. Byzantium did not possess much of a navy until the 5th c. The Byzantine Empire was vast, however, and in order to meet the needs of all its provinces it required a large merchant navy. Justinian (527-565 AD) understood this necessity, as well as the great importance of the sea for transport and communications, and he set about organising a fleet. Thus Byzantium slowly became the dominant sea power of the time, with the Aegean playing a leading part. The new capital, Constantinople, became the biggest centre of commercial activity. The large ships that had been built, chiefly for transporting cargoes of grain from Alexandria to Rome, began to be used less frequently, because most of these cargoes now went to Constantinople, and for the voyage through the Aegean to the Golden Horn the ships had to be smaller and more manoeuvrable. Furthermore the big ships were slow-moving and not suited to evading sudden attacks by pirates, who constituted a real plague for shipping at the time. These new requirements led to the construction of smaller, swifter vessels, which came to play an important role in the maritime trade of the Aegean during the Byzantine period. At this time there appeared the fast, light mobile ship known as the dorkon (from the ancient word dorkas which means "roe-buck") a vessel of some 130-140 tons capacity with lateen sails, which gave better steering and manoeuvrability. In the same century (6th) the dromon appeared, a new type of fast, light vessel with lateen sails and a protective deck above the rowers. The dromons were chiefly warships, but they were also used as merchantmen on long voyages. The smaller dromon was known as a chelandiοn the larger one as a meizon dromon and the flagship as the pamphylos dromon.
At the end of the 6th c. the State adopted the system of chartering ships from private owners, who during the 7th c. became an important class that was very influential in Byzantine society, since they played a big part in the imperial economy. These ship owners were also often captains and traders themselves (they were known later on in the Aegean as karavokyrides). This was the time when the maritime law was codified and many beneficial and protective measures were taken for sea-trade.
The amount of real information available about the merchantman of the Byzantine period is unfortunately slight and consists mainly of written texts. "No records tell us about the Byzantine ships", said Professor Rados at the beginning of the 20th century. Recently, however, additional evidence has come from two wrecks of merchantmen, of the 4th and 7th c. AD, which were discovered at the islet of Yassi Ada (between the Turkish coasts and the island of Pserimos in the Dodecanese) and raised with part of their cargo. From the drawings of the 7th c. ship made by the archaeologists, it appears that this vessel resembled both the ancient holkas and the modern trechandiri. These, too, are pieces of the thread connecting the different phases of the development of merchant shipping in the Aegean. Byzantine maritime supremacy began to wane progressively from the time of the Crusades and especially after the l2th c. Furthermore, the rise of the city-states of Italy, particularly Venice and Genoa, and their growth into important maritime powers resulted in the Byzantines losing their domination in the Aegean, which gradually fell mainly under the control of the Venetians. These changes had a catastrophic effect on the whole of the nautical world in the region. The Greek shipmasters and sailors of the Aegean found themselves faced with a host of obstructions and difficulties, which were accentuated by religious fanaticism and piracy on the part of both Easterners and Westerners.