During the two centuries of Roman rule, Getian insurgents, Goths, and Sarmatians harassed Dacia, and by the middle of the third century A.D. major migrations of barbarian tribes had begun. In 271 A.D. Emperor Aurelian concluded that Dacia was overexposed to invasion and ordered his army and colonists to withdraw across the Danube. Virtually all the soldiers, imperial officials, and merchants departed; scholars, however, presume that many peasants remained. Those Dacians who departed spread over the Balkans as far as the Peloponnese, where their descendants, the Kutzovlachs, still live.

Without Rome's protection, Dacia became a conduit for invading tribes who, targeting richer lands further west and south, plundered Dacian settlements in passing. Dacian towns were abandoned, highwaymen menaced travelers along crumbling Roman roads, and rural life decayed. The Visigoths, Huns, Ostrogoths, Gepids, and Lombards swept over the land from the third to the fifth centuries, and the Avars arrived in the sixth, along with a steady inflow of Slavic peasants. Unlike other tribes, the Slavs settled the land and intermarried with the Dacians. In 676 the Bulgar Empire absorbed a large portion of ancient Dacia.

The migration period brought Dacia linguistic and religious change. The Dacians assimilated many Slavic words into their lexicon and, although modern Romanian is a Romance language, some linguists estimate that half of its words have Slavic roots. Baptism of the Dacians began around 350 A.D. when Bishop Ulfilas preached the Arian heresy north of the Danube. Soon after saints Cyril and Methodius converted the Bulgars to Christianity in 864, Dacia's Christians adopted the Slavonic rite and became subject to the Bulgarian metropolitan at Ohrid. The Slavonic rite would be maintained until the seventeenth century, when Romanian became the liturgical language.

Data as of July 1989