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Genjutsu (幻術; Literally meaning "Illusionary Techniques") are techniques that are employed in the same fashion as ninjutsu, requiring chakra and hand seals. However, the primary difference between the two is that the effects of genjutsu are illusory; instead of attacking the victim's body, like taijutsu or ninjutsu, genjutsu techniques manipulate the flow of chakra in the victim's brain, thus causing a disruption in their senses.
This is often used to create false images or to cause pain from trauma (because the body is led to believe it is in pain); however, there are plenty of other uses depending on the situation. They are not used as often as ninjutsu because they don't physically harm the opponent, but several characters have made themselves well-known for their great prowess with genjutsu like Itachi Uchiha, Kurenai Yūhi, and Tayuya. Genjutsu can also be used to manipulate others, similar to brainwashing by feeding the victim illusive suggestions, the primary example being Madara Uchiha controlling Yagura. However, there exists at least one genjutsu, Izanagi, that has the opposite effects to normal genjutsu, by changing reality to illusion and vice versa.
Genjutsu is created when a ninja extends their chakra flow through the cerebral nervous system of their opponent to control their mind's chakra, thereby affecting their five senses. Those with special abilities like the Uchiha clan's Sharingan or with high intelligence have an easier time executing and countering genjutsu, as attention to detail is key. Those under the influence of genjutsu either freeze in place or lose consciousness, depending on how capable they are in recognizing and defending against it.
Genjutsu attacks usually require a lot of skill, and, if performed incorrectly, will not work; even Jiraiya had trouble performing such techniques. Genjutsu is most often used by ninja with very precise chakra control and very high intelligence, the most notable being Itachi Uchiha.
Because genjutsu affects activity throughout the Prosencephalon structure of the brain, humans, dogs and other species of mammal are therefore all affected by genjutsu; however, the kikaichū, like all insects, lack such a structure to their brains, and so do not even notice it: as Shino Aburame put it, insects are ruled almost solely by five powerful senses alone, which makes them immune to genjutsu and gives the Aburame clan an ability to detect it when nobody else can.
Shikamaru Nara states that among genjutsu the most dangerous are those that affect the opponent with sound allowing the user to stay out of sight. Tayuya is a great example of this sound-based, long-range type of combat.
To combat the effects of genjutsu, there are several options:
The first option is for the ninja to stop the flow of chakra in their body, and then apply an even stronger power to disrupt the flow of the caster's chakra; this is called Genjutsu Release (幻術解, Genjutsu Kai). This can also be done by an unaffected ninja by applying a sudden surge of chakra into the affected person. In addition, tailed beasts can break their jinchūriki out of genjutsu in a similar fashion if they have a good enough cooperation, as seen in Killer B's case.
The second option is through intense pain not caused by the genjutsu and utilizing the resulting pain to bring their senses back in order (of the first two options, the latter seems more effective in most situations, or at least is the easiest).
The third option is accessible only to Sharingan wielders. Members of the Uchiha clan can break or see through genjutsu using their Sharingan. Breaking Tsukuyomi is possible with an ordinary Sharingan, but requires a lot of skill.
Fourthly, ordinary people can sometimes train their eyes (without Sharingan) to break genjutsu through unknown means, like Deidara.
Finally, one can prevent a person from casting genjutsu using obstructive techniques like the Hidden Mist Technique to prevent one from seeing the opponent in the first place, as Zabuza demonstrated against Kakashi.
In spite of these methods, some forms of genjutsu like Itachi's Tsukuyomi and Shima and Fukasaku's Demonic Illusion: Toad Confrontation Singing may not be broken easily, either due to their near-instant effect like the former, or their ability to render one unconscious and paralysed altogether like the latter. In addition, one must realize that he or she is under the effects of genjutsu before attempting to break out of it. This can be done by observing the changes in one's own chakra like Karin's chakra-sensing ability.