Kinjutsu

are techniques that have been banned from being taught or used. The term first came into use following the end of the Warring States Period, when the newly-formed hidden villages agreed to forbid certain kinds of techniques in the interests of reducing unnecessary bloodshed and maintaining the peace. Although the hidden villages no longer teach kinjutsu to their ninja, they have preserved the knowledge of how to perform them: in Konohagakure, instructions for the Multiple Shadow Clone Technique are detailed in the Scroll of Seals; Takigakure "prized" the Earth Grudge Fear, but it was stolen by Kakuzu; Deidara similarly stole the kinjutsu used with his Explosive Clay from Iwagakure; Sunagakure actively promoted creation of what became One's Own Life Reincarnation, but halted all research once it became apparent the technique would qualify as kinjutsu.

Despite being forbidden, it's unclear what the actual penalties for using kinjutsu are, if any. Naruto Uzumaki regularly performs the Multiple Shadow Clone Technique without reprimand, though this may be because he is one of the few who can perform it safely. When the Fifth Hokage determines that Naruto's Wind Release: Rasenshuriken meets the definition of kinjutsu, Naruto is only encouraged not to use it because of the risks the technique carries. Kakuzu and Deidara are deemed S-rank missing-nin, but this presumably has less to do with their use of kinjutsu than it does betraying their villages in order to steal the kinjutsu in the first place. Orochimaru is also deemed an S-rank missing-nin, though his crimes stem from his actions while developing the Living Corpse Reincarnation rather than from its actual use. Conversely, the Fourth Hokage's use of the Dead Demon Consuming Seal earned him the label of a hero because he used it to protect Konoha, a purpose that the Third Hokage also believes justifies his own use of it.

There are several different reasons that can lead to a technique's classification as kinjutsu:
 * Techniques that endanger the user's life, such as Twin Snakes Mutual Death Technique and the Reverse Lotus.
 * Techniques with severe chakra requirements to perform, such as the Multiple Shadow Clone Technique.
 * Techniques that violate the laws of nature, such as Summoning: Impure World Reincarnation.
 * Techniques that cost the life of the user and/or another sacrifice, such as One's Own Life Reincarnation.
 * In the case of Izanagi, the Uchiha clan declared it a kinjutsu because its use led the blinding of the user's Sharingan, a kekkei genkai that the Uchiha prized.
 * Techniques which cause destruction on a large scale, such as the anime-exclusive Fury.

Trivia

 * Despite not being an actual technique, the Eight Gates has been called a forbidden technique.
 * In the anime, in exchange for Fury being classified as kinjutsu, the Third Hokage agreed that Konohagakure would aid the Tsuchigumo Clan in time of danger.