The Slav invasion and settlement of the Balkans can be divided into three phases. During the first, covering the first half of the sixth century, the Slavs were still based north of the Danube but kept up a constant pressure of raids across the river, which yielded them plunder, slaves, and bribes to remove themselves. They were behaving much as other barbarian peoples before them and the Byzantine authorities reacted predictably. There were as yet few attempts to make permanent lodgements south of the Danube. The events of this half-century are treated in Prokopios's Gothic War. A few landmarks must suffice here. The wide-ranging movements of the Getae in the North Balkans from 517 may have brought Slavs in as contingents in their armies. Prokopios alludes to a large-scale Slav raid on Thrace about 527. In the 530s there were further substantial incursions and in the 540s massive raids, which at one moment menaced Constantinople itself. In 547–8 a great offensive reached the Adriatic coast and devastated Dyrrachium (Durazzo). In the 550s the Imperial City was again menaced; this time the Slavs were strengthened by Kutrigur Turks.

As usual, the Empire made some attempt to tame the barbarians by attracting them as mercenary contingents into its armies and employing them on other war fronts: as early as 536–7 we find a record of such Slav mercenaries fighting against the Ostrogoths in Italy.