Ships of various kinds and tonnage were included in the property, owned and traded by a considerable number of monastic communities in the Byzantine era. These ships, commercial as they were, had an exporting and importing function, thus serving both the production and consumption of the monasteries. Due to their important role they didn’t have to pay harbour or customs fees and even enjoyed tax exemption. The official documents preserved in the archives of the ship-owner monasteries serve as our main source of information on all matters relating to these commercial ships. Most important among them are the documents in the keeping of the monastery of St. John the Theologian on Patmos. Their significance lies in two main facts. First, more than half of them have been published in an excellent edition, and thus they are easily accessible to the scholars of the field. Secondly, they include three acts dealing with the commercial navy, that can be considered as unique, not only because of their content, but also for their diplomatic value. It should be noted, in addition, that the St. John the Theologian monastery, dominating the entire island of Patmos, was already since its foundation a ship-owner institution. The monk Christodoulos, founder of the monastery, had owned and bequeathed through his will four commercial ships to the monastery.