Spread of Islam
Ottoman Islamic standards of toleration allowed for autonomous "nations" ("millets") in the Empire, under their own personal law and under the rule of their own religious leaders. As a result, vast areas of the Balkans remained mostly Christian during the period of Ottoman domination. In fact, the Eastern Orthodox Churches had a higher position in the Ottoman Empire, mainly because the Patriarch resided in Istanbul and was an officer of the Ottoman Empire. In contrast, Roman Catholics, while tolerated, were suspected of loyalty to a foreign power (the Papacy). It is no surprise that the Roman Catholic areas of Bosnia, Kosovo and northern Albania, ended up with more substantial conversions to Islam. The defeat of the Ottomans in 1699 by the Austrians resulted in their loss of Hungary and present-day Croatia. The remaining Muslim converts in both elected to leave "lands of unbelief" and moved to territory still under the Ottomans. Around this point in time, new European ideas of romantic nationalism started to seep into the Empire, and provided the intellectual foundation for new nationalistic ideologies and the reinforcement of the self-image of many Christian groups as subjugated peoples.
As a rule, the Ottomans did not require followers of Greek Orthodoxy to become Muslims, although many did so in order to avert the socioeconomic hardships of Ottoman rule 
One by one, the Balkan nationalities asserted their independence from the Empire, and frequently the presence of members of the same ethnicity who had converted to Islam presented a problem from the point of view of the now dominant new national ideology, which narrowly defined the nation as members of the local dominant Orthodox Christian denomination. Some Muslims in the Balkans chose to leave, while many others were forcefully expelled to what was left of the Ottoman Empire. This demographic transition can be illustrated by the decrease in the number of mosques in Belgrade, from over 70 in 1750 (before Serbian independence in 1815), to only three in 1850.
Turkish people
Under its founder, Osman I, the nomadic Ottoman beylik expanded along the Sakarya River and westward towards the Sea of Marmara. Thus, the population of western Asia Minor had largely become Turkish-speaking and Muslim in religion. It was under his son, Orhan I, who had attacked and conquered the important urban center of Bursa in 1326, proclaiming it as the Ottoman capital, that the Ottoman Empire developed considerably. In 1354, the Ottomans crossed into Europe and established a foothold on the Gallipoli Peninsula while at the same time pushing east and taking Ankara. Many Turks from Anatolia began to settle in the region abandoned by the inhabitants who had fled Thrace before the Ottoman invasion. However, the Byzantines were not the only ones to suffer from the Ottoman advancement for, in the mid-1330s, Orhan annexed the Turkish beylik of Karasi. This advancement was maintained by Murad I who more than tripled the territories under his direct rule, reaching some , evenly distributed in Europe and Asia Minor. Gains in Anatolia were matched by those in Europe; once the Ottoman forces took Edirne (Adrianople), which became the capital of the Ottoman Empire in 1365, they opened their way into Bulgaria and Macedonia in 1371 at the Battle of Maritsa. With the conquests of Thrace, Macedonia, and Bulgaria, significant numbers of Turkish emigrants settled in these regions. This form of Ottoman-Turkish colonization became a very effective method to consolidate their position and power in the Balkans. The settlers consisted of soldiers, nomads, farmers, artisans and merchants, dervishes, preachers and other religious functionaries, and administrative personnel.