This study discusses some important questions and historical processes concerning the history of Serdica within the boundaries of the Eastern Roman Empire and the Early Byzantium. The author comes to the conclusion that the geographical situation of the town and its human resources turned it into a strategic point in the contacts of the Byzantine Empire with the peoples and tribes that inhabited the territories to the north-west of its frontiers and in the Empire's relations with the other great religious centre of medieval Europe - the Papacy.