Janissaries, created by Sultan Murad I, were young Christian boys from conquered countries (commonly Greek), who were converted to Islam and levied into the Devshirme system (or the "Collection of Children"). They were also nicknamed "slave soldiers", because of their method of capture. The strongest of the children were usually chosen to become Janissaries, whereas the more intelligent ones would become scientists or architects. 
The Janissaries became the Ottoman Empire's first standing army, and were required to learn and convert to Islam. This elite sect of soldiers were also the mainstay of the Ottoman army during the 1453 invasion of Constantinople, led by Sultan Mehmed II. They were essentially infantry units that formed the Sultan's household troops and bodyguard. However, after the 17th century, they began to falter in importance and strength until they were eventually abolished by Sultan Mahmud II in 1826, as a result of the Auspicious Incident. 
In the 16th century, the Janissaries were led by Tarik Barleti. Due to the habit of some members terrorizing the populace of Constantinople, they were widely feared by the civilians. The arrogant habits of these Janissaries - destroying vendors' booths, confiscating products, and so on - raised the ire of many citizens who saw them as being above the law.
The Assassin Ezio Auditore da Firenze used this to his advantage by paying heralds to incite a riot in the Harbor of Theodosius, enabling him to sneak into the harbor and spy on a meeting between their commander, Tarik, and the Byzantines Manuel Palaiologos and Shahkulu.
Under Prince Suleiman's orders, Ezio lured a Janissary into a hiding spot and killed him, and then took his clothes in order to infiltrate the Janissary camp in the Constantine District, although the body was soon found and the camp was put on alert. When Ezio assassinated Tarik, he discovered that he was not a traitor, merely secretly trying to infiltrate the Byzantine Templars. After Ezio escaped, the Janissaries began patrolling all of Constantinople and raised the Great Chain to prevent his departure.
However, this ultimately failed, as Ezio destroyed the Great Chain with a bomb, and sank the ships blockading the Golden Horn by spraying them with Greek Fire. He then left Constantinople for Derinkuyu on a chartered ship, and when he returned, the Janissaries were on high alert, with them declaring a warrant for his capture.
- Janissaries could not be instantly killed by counter-attacks and execution streaks from full health. However, they still took damage whenever one of the two was performed.
- Despite Janissaries not being initially vulnerable to execution streaks, combo chains could still be continued by attacking another guard, and could also be started by counter-stealing from a Janissary instead.
- Like Almogavars, all Janissaries wielded the same sword: the Janissary Kijil.
- On some occasions, if Ezio was lacking in lethal or tactical bombs, looting or stealing from Janissaries would give him splinter or caltrop bombs.
- Janissaries were very resistant to gunfire, but were easily killed by bombs and poison.
- In the novelization of Assassin's Creed: Revelations, the Janissaries were depicted as wearing white armor instead of green.
- Janissaries were faster than Ezio, and could climb onto most buildings, which made them difficult to escape from.
- Their name was originally a compound of Turkish words yeni, "new" and çeri, "army."