Among those who were responsible for the projection of Ottoman power, few deserve as much credit as the Janissaries. Established by Murad I, the Janissaries became an elite infantry force that was loyal only to the sultan. Their mission was to protect only him and in battles they were always the closest to him, forming a human shield. Originally, they consisted of non-Muslim slaves, mainly Christian boys from Byzantium. Jewish boys were not taken as soldiers and Muslims could not, by law, be enslaved. Murad had instituted a tax of one fifth on all the slaves taken in war, and the idea of only taking boys fit for fighting was called Devshirme, or blood tax. The slaves went through a very strict training, first learning to speak Turkish and practicing Ottoman traditions by living with a family chosen by the sultan. The boys also were forcibly converted to Islam, forbidden from wearing a beard and lived under monastic circumstances in celibacy. They were overseen by eunuchs and trained in special schools, enhancing their personal abilities. The main difference between these and other slaves was that they were being paid for their services. This served as a motivator and kept the soldiers loyal.

The Janissary corps was the first of its kind and a groundbreaking contributor to the success of Ottoman warfare. At the time of Murad's reign, they were fewer and less respected than what they would become at a later stage, but they were quite significant for Ottoman victories in the Balkans, and as such, they became notorious in Europe. One of the first major battles in which the Janissaries participated occurred when Murad's successor, Bayazid, managed to inflict a decisive defeat on a European army near Nikopol. In general, the Janissaries were positioned at the front lines of the Ottoman army and were armed with various weapons, including bows and arrows, swords, spears, and shields.