Genjutsu (幻術, Literally meaning: Illusionary Techniques) is one of the main jutsu categories which uses chakra. Unlike ninjutsu, the effects of genjutsu are not real, being only illusions by those who fall victim to it. According to the Second Mizukage, genjutsu falls under the broad category of Yin Release.
A genjutsu is created when a ninja controls the chakra flow of a target's cerebral nervous system, thereby affecting their five senses. This is frequently used to create false images and/or trick the body into believing its has experienced physical pain. Genjutsu can also be used to manipulate others - similar to brainwashing - by feeding the victim illusive suggestions. Most genjutsu is performed visually, requiring the target make eye contact with the caster. Other methods include integration with nature transformation, as shown with Water and Lightning Release, and in the anime, ingesting special pills that activate when the target sleeps. Audible genjutsu is deemed the most dangerous because the user can trap victims from long distances.
Despite its usefulness, genjutsu is rarely employed, most shinobi preferring the real, tangible effects of ninjutsu and taijutsu over the imagined effects of genjutsu. The difficulty of performing genjutsu in the first place presents an additional barrier to its use: genjutsu require precise chakra control in order to manipulate the target's senses and advanced intelligence to make the illusion convincing, prerequisites that even someone as skilled as Jiraiya lacks. Some, however, are able to specialise in genjutsu, with Kurenai Sarutobi being regarded as one of Konoha's experts in the field and members of the Uchiha clan being particularly feared because of their Sharingan's generic genjutsu. According to Hagoromo, the Byakugan can also be used to extend the range of genjutsu.
Those under the influence of genjutsu will either appear to be unconscious or continue to move around under the pretence of the genjutsu. The latter is very effective in battle as a genjutsu user can make themselves appear to be somewhere else and attack the victim from behind or change the environment to make the victim see a flat plain where there is a steep cliff.