The GreatBulgaria in Roman times had been called Moesia and had a mixed population of Thracians, Greeks and Dacians, most of whom spoke either Greek or a sub-Latin language known as Romance. It had been overrun by the Slavs in the mid 7th century. In 681 the Bulgars founded a Khanate on the Danube after the defeating the Byzantine army under Emperor Justinian II in a battle south of the Danube delta. Following their defeat an agreement was made between the Bulgar ruler Asparukh and the Byzantine Emperor. The agreement gave the Bulgars the territorry between the Danube and the Balkan. The Byzantines also agreed to pay a yearly tax to the Khanate, a usual Byzantine custom. The Bulgars were greatly outnumbered by the Slav population among whom they had settled. Between the 7th and the 10th centuries, the Bulgars were gradually absorbed by the Slavs, adopting a South Slav language and converting to Orthodox Christianity under Boris I in 865. By 1000, the Bulgars had become Bulgarians, who are now classed as a South Slav people related to the Serbs, rather than a Turanian people. The Bulgar Khan became the Czar of Bulgaria, and the Bulgarian Empire (called by some historians the West Bulgarian Empire to distinguish it from the lands of the Turanian Bulgars who still lived in the Volga valley).

The arrival of the Magyars in Hungary in the 9th century forced the Bulgars out of this area, enouraging them to expand to the south. Under the warrior Khan Krum (802-14, also known as Crummus and Keanus Magnus), Bulgaria expanded southwards, occupying Sofia in 811 and Adrianople (modern Edirne) in 813, and threatening Constantinople itself. Under Boris I the Bulgarians became Christians, and the Ecumenical Patriarch agreed to allow an independent Bulgarian Patriarch at Ochrida.

Missionaries from Constantinople, Cyril and Methodius, devised the Glagolitic alphabet, adopted in the Bulgarian Empire around 886. The alphabet and the Old Church Slavonic language gave rise to a rich literary and cultural activity centered around the Preslav and Ohrid Literary Schools, established by order of Boris I in 886. In the beginning of 9th century AD, a new alphabet - the Cyrillic alphabet - developed on the basis of Greek and Glagolitic cursive at the Preslav Literary School. According to an alternative theory, the alphabet was devised at the Ohrid Literary School by Saint Climent of Ohrid, a Bulgarian scholar and disciple of Cyril and Methodius. A pious monk of Sofia, St Ivan of Rila (Ivan Rilski, 876-946), became the patron saint of Bulgaria.

By the late 9th century Bulgaria extended from the mouth of the Danube to Epirus in the south and Bosnia in the north-west. A Serbian state came into existence as a dependency of the Bulgarian Empire. Under Czar Simeon I (Simeon the Great), who was educated in Constantinople, Bulgaria became a serious threat to the Byzantine Empire. Simeon hoped to take Constantinople and make himself Emperor of both Bulgars and Greeks, and fought a series of wars with the Byzantines through his long reign (893-927). Simeon proclaimed himself "Tsar (Caesar) of the Bulgarians and the Greeks," a title which was recognised by the Pope, but not of course by the Byzantine Emperor. Simeon made Sofia a centre of learning.

After Simeon's death, however, Bulgarian power declined. Under Peter I and Boris II the country was divided by the egalitarian religious heresy of the Bogomils, and distracted by wars with the Hungarians to the north and the breakaway state of Serbia to the west. In 972 Emperor John Tsimisces was able to make eastern Bulgaria a Byzantine protectorate. The Bulgarians maintained an independent state for a time in the western part of the country, but in 1014 Emperor Basil II defeated the armies of Czar Samuil at the Balasita and massacred thousands, acquiring the title "Bulgar-slayer" (Voulgaroktonos). He ordered 14,000 Bulgarian prisoners blinded and sent back to their country. At the sight of his returning armies Samuil suffered a heart attack and died. By 1018 the country had been mostly subjugated by the Byzantines.