The city of Erzurum, known throughout history by different names such as Karin, Theodosiopolis, Kālîkalâ, Erzenü’r-Rûm, Arz-ı Rûm, was founded under the name of Theodosiopolis in the first quarter of the 5th century as a fortified position against Iranian attacks. The city, which changed hands first between Byzantium and Iran and then between Byzantium and Arabs until the 10th century, came under Byzantine rule in 949, which lasted until the Turkish conquest. A number of political and socio-economic changes have occurred in the city, both during the previous border struggles and during the last period of its long stay under Byzantine rule. First of all, the Byzantine state established a new military-administrative management system in the areas it recaptured along its entire eastern border. Within the framework of this new change, he organized Erzurum as a small thematic unit and turned it into an administrative center. On the other hand, before the Turkish conquest, Erzurum and its surroundings were very active economically. The city was an important caravan city, apparently trading with Georgians in the early tenth century. XI. century Byzantine and Armenian sources report that the city was very populous and rich and that many merchants from Syrian, Armenian and other nations resided in the city. In addition, new conquests also changed the demographic structure on the eastern border of the state. The regions desolated as a result of Byzantine attacks were resettled with different ethnic groups. However, the incompatibility between the belief of this new Christian population and the official beliefs of the empire constituted another problem. In this study, we will try to determine the changes in the administrative and socio-economic situation of the region in the period until its conquest by the Turks, in the light of the sources.