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, architects, or government officials. 
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.
- The name "Yeniçeri" is a compound of the Turkish words yeni, "new" and çeri, "army"; an alternate translation could read as "new soldiers".
- In the novelization of Assassin's Creed: Revelations, the Janissaries are described as wearing white armor. This is not evident in the game where their armor is covered by their embroidered uniforms.
- Out of all elite units in the Assassin's Creed series, Janissaries are the most regular; after the completion of Sequence 7, with Ezio Auditore returning to Constantinople from Cappadocia, every guard patrol in the city will consist of two Janissaries.
- Janissaries cannot be instantly killed by counter-attacks and execution streaks from full health. However, they still take damage whenever either of the two are performed.
- Despite Janissaries not being initially vulnerable to execution streaks, combo chains can still be continued by attacking another guard and can also be started by counter-stealing them instead.
- Janissaries are very resistant to gunfire but are easily killed by bombs and poison.
- On some occasions, if Ezio is lacking in lethal or tactical bombs, looting or stealing from Janissaries give him splinter or caltrop bombs.
- Janissaries are faster than Ezio and can climb onto most buildings which make them difficult to escape from.
- All Janissaries wield the same sword: the Janissary Kijil.
- Unlike the Papal Guard who follow-up every shot with a strafing maneuver, Janissaries follow-up by dashing forwards to kick their target.
- Strangely, Janissaries are especially vulnerable to unarmed combat. Performing a counter-steal while unarmed will immediately open them up to an execution, and they also struggle to block against attacks.
- Attempting a hook and run on a Jannisary will cause the hookblade to be temporarily caught in their armor.
- Assassin's Creed: Revelations
- Assassin's Creed: Revelations novel
- Assassin's Creed: Revelations – Discover Your Legacy