The great Slavonic immigration, which changed the ethnographic face of the Peninsula, began in the 3rd century A.D. and continued at intervals throughout the following four centuries. At the beginning of this movement the Byzantine empire was in actual or nominal possession of all the regions south of the Danube; the greater part of the native Thraco-Illyrian population of the interior had been romanized and spoke Latin. The Thracians, the progenitors of the Vlachs, took refuge in the mountainous districts and for some centuries disappeared from history: originally an agricultural people, they became nomad shepherds. In Albania the aboriginal Illyrian element, which preserved its ancient language, maintained itself in the mountains and eventually forced back the immigrant race. The Greeks, who occupied the maritime and southern regions, were driven to the sea-coast, the islands and the fortified towns. Slavonic place-names, still existing in every portion of the Peninsula, bear witness to the multitude of the invaders and the permanency of their settlements. In the 6th century the Slavs penetrated to the Morea, where a Slavonic dialect was spoken down to the middle of the 15th century.

In the 7th the Serbo-Croats invaded the north-western regions (Croatia, Servia, Bosnia, Herzegovina, Montenegro and northern Albania); they expelled or assimilated the Illyrian population, now represented in Dalmatia by the slavonized Morlachs or Mavro-Vlachs, and appropriated the old Roman colonies on the Adriatic coast. At the end of the 7th century the Bulgars, a Turanian race, crossed the Danube and subjected the Slavonic inhabitants of Moesia and Thrace, but were soon assimilated by the conquered population, which had already become partly civilized. Under their tsar Krum (802-815) the Bulgars invaded the districts of Adrianople and central Macedonia; under Simeon (893-927), who fixed his capital at Preslav, their empire extended from the Adriatic to the Black Sea. In 971 "the first Bulgarian empire" was overthrown by the emperor John Zimisces, but Bulgarian power was soon revived under the Shishman dynasty at Ochrida. In 1014 Tsar Samuel of Ochrida, who had conquered the greater part of the Peninsula, was defeated at Belasitza by the Greek emperor Basil II., and the "western Bulgarian empire" came to an end.

In the 10th century the Vlachs reappear as an independent power in Southern Macedonia and the Pindus district, which were known as Great Walachia (μεγάλη βλαχία). The Serbs, who owing to the dissensions of their zhupans or chiefs, had hitherto failed to take a prominent part in the history of the Peninsula, attained unity under Stephen Nemanya (1169-1195), the founder of the Nemanyich dynasty. A new Bulgarian power, known as the "second" or "Bulgaro-Vlach empire," was founded at Trnovo in 1186 under the brothers Ivan and Peter Asên, who led a revolt of Vlachs and Bulgars against the Greeks. In 1204 Constantinople was captured by the Latins of the Fourth Crusade, and Baldwin of Flanders was crowned emperor; the Venetians acquired several maritime towns and islands, and Frankish feudal dynasties were established in Salonica, Athens, Achaea and elsewhere. Greek rule, however, survived in the despotate of Epirus under princes of the imperial house of the Angeli. The Latin tenure of Constantinople lasted only 57 years; the imperial city was recaptured in 1261 by Michael VIII. Palaeologus, but most of the feudal Latin states continued to exist till the Turkish conquest; the Venetians retained their possessions for several centuries later and waged continual wars with the Turks. In 1230 Theodore of Epirus, who had conquered Albania, Great Walachia and Macedonia, was overthrown at Klokotnitza by Ivan Asên II., the greatest of Bulgarian monarchs (1218-1241), who defeated Baldwin at Adrianople and extended his sway over most of the Peninsula. The Bulgarian power declined after his death and was extinguished at the battle of Velbûzhd (1330) by the Servians under Stephen Urosh III. A short period of Servian predominance followed under Stephen Dushan (1331-1355) whose realm included Albania, Macedonia, Epirus, Thessaly and northern Greece. The Servian incursion was followed by a great Albanian emigration to the southern regions of the Peninsula. After Dushan's death his empire disappeared, and Servia fell a prey to anarchy.

For a short time the Bosnians, under their king Stephen Tvrtko (1353-1391), became the principal power in the west of the Peninsula. The disorganization and internecine feuds of the various states prepared the way for the Ottoman invasion. In 1356 the Turks seized Gallipoli; in 1361 the sultan Murad I. established his capital at Adrianople; in 1389 the fate of the Slavonic states was decided by the rout of the Servians and their allies at Kossovo. The last remnant of Bulgarian national existence disappeared with the fall of Trnovo in 1393, and Great Walachia was conquered in the same year. Under Mahommed II. (1451-1481) the Turks completed the conquest of the Peninsula. The despotate of Epirus succumbed in 1449, the duchy of Athens in 1456; in 1453 Constantinople was taken and the decrepit Byzantine empire perished; the greater part of Bosnia submitted in 1463; the heroic resistance of the Albanians under Scanderbeg collapsed with the fall of Croia (1466), and Venetian supremacy in Upper Albania ended with the capture of Scutari (1478). Only the mountain stronghold of Montenegro and the Italian city-states on the Adriatic coast escaped subjection.

In the 16th century under Solyman the Magnificent (1520-1566) the Ottoman power attained its greatest height; after the unsuccessful siege of Vienna (1683) it began to decline. The period of decadence was marked in the latter half of the 18th century by the formation of practically independent pashaliks or fiefs, such as those of Scutari under Mahommed of Bushat, Iannina under Ali of Tepelen, and Viden under Pasvan-oglu. The detachment of the outlying portions of the empire followed. Owing to the uncompromising character of the Mahommedan religion and the contemptuous attitude of the dominant race, the subject nationalities underwent no process of assimilation during the four centuries of Turkish rule; they retained not only their language but their religion, manners and peculiar characteristics, and when the power of the central authority waned they still possessed the germs of a national existence. The independence of Greece was acknowledged in 1829, that of Servia (as a tributary principality) in 1830. No territorial changes within the Peninsula followed the Crimean War; but the continuance of the weakened authority of the Porte tended indirectly to the independent development of the various nationalities.