Genjutsu

is one of the main jutsu  categories which uses chakra. Unlike ninjutsu, the effects of genjutsu are not real, being only sensory illusions experienced by those who fall victim to it. According to the Second Mizukage, genjutsu falls under the broad category of Yin Release.

Overview
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. It is further explained in the anime that, 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 their senses alone, which makes them immune to genjutsu and gives the Aburame clan an ability to detect it, even when heightened senses become susceptible.

Genjutsu can also be used to manipulate others - similar to brainwashing - by feeding the victim illusive suggestions. 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 which hold little influence over the physical world. 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 Yūhi 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. Alternatively, a genjutsu user can create relatively small differences in the environment, which is more effective for intel gathering. For example, Itachi Uchiha was known to cast a genjutsu upon meeting someone, and that person's reaction to Itachi's image in the genjutsu would help him understand them better while keeping himself safe.

Countering
There are several options to combat genjutsu:
 * A victim's chakra flow, if disrupted enough, can break the caster's influence. This or  can be done by the victim themselves, (assuming they realise they're under a genjutsu's influence) or an ally who emits chakra into them; jinchūriki who have formed a good relationship with their tailed beast prefer the latter method whenever they need it.
 * Pain that is not caused by the genjutsu can bring victims back to their senses.
 * If a caster's vision is obstructed, such as by the Hiding in Mist Technique, that at least prevents the use of ocular genjutsu.
 * Possessing a Sharingan renders one largely invulnerable to genjutsu as their acute vision allows them to recognise its flaws and influence. They can also free others from genjutsu by inserting themselves into the opponent's illusion and release it.
 * Deidara is somehow able to condition his left eye to break genjutsu.
 * In the anime, kikaichū are impervious to genjutsu because their brain structures aren't sophisticated enough, allowing members of the Aburame clan a method to identify and counter genjutsu.
 * In the anime, strong smells, such as those from Chilli Pepper Bombs, can counteract genjutsu.
 * In the anime, Kabuto Yakushi attempted to use his own genjutsu on himself in order to override the one Itachi Uchiha placed on him, though due to the complex nature, it failed.

Trivia

 * Izanagi has the opposite effects to normal genjutsu, changing reality to illusion and vice versa.
 * Obito Uchiha had been able to completely control Yagura Karatachi with a genjutsu, despite the latter being a perfect jinchūriki.
 * In the anime, genjutsu even works on Mecha-Naruto, corrupting its internal system.
 * Because genjutsu are not real, they are not supposed to be able to kill victims. However, Izumi Uchiha is killed with Tsukuyomi in Itachi Shinden: Book of Dark Night.
 * In the anime, due to the Kurama Clan's kekkei genkai, certain members can use their exceptional talent for genjutsu to make the brain to believe the illusions are physically real, which can cause the victim to be potentially killed.
 * When Ao dispelled the genjutsu that Obito Uchiha placed on Yagura Karatachi, the after effects ended up killing the latter.
 * Itachi claimed that his Tsukuyomi can only be broken by a Sharingan user who shares the same blood with him.