Hosios Loukas (Ὅσιος Λουκᾶς) is a historic walled monastery situated near the town of Distomo. It is one of the most important monuments of Middle Byzantine architecture and art, and has been listed on UNESCO World Heritage Site, along with the monasteries of Nea Moni and Daphnion. It was founded in the early 10th century AD by the hermit, the Venerable St. Luke, whose relics are kept in the monastery to this day.
The Church of the Theotokos, the oldest in the complex, is the only church known with certainty to have been built in mainland Greece in the 10th century. This centralized parallelogram-shaped building is the oldest example of the cross-in-square type in the country; its plan closely follows that of Lips Monastery in Constantinople. The walls are opus mixtum (part brick, part stone, part marble) and display curious pseudo-kufic patterns.
The Church of the Theotokos adjoins a larger cathedral church, or Katholikon, tentatively dated to 1011-1012. The Katholikon is the earliest extant domed-octagon church, with eight piers arranged around the perimeter of the nave. The hemispherical dome rests upon four squinches which make a transition from the octagonal base under the dome to the square defined by the walls below. The main cube of the church is surrounded by galleries and chapels on all four sides.