Genjutsu

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 Furudokage, 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. 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 Tristan 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 succeptible.

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.

Countering
There are several options to combat genjutsu:
 * A victim's chakra flow, if disrupted enough, can break the caster's influence. This "Genjutsu Dissipation" or "Genjutsu Reversal" 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.
 * Kenji Nara is somehow able to condition his left eye to break genjutsu.
 * 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.
 * Strong smells, such as those from Chilli Pepper Bombs, can counteract genjutsu.
 * Ace Yakushi attempted to use his own genjutsu on himself in order to override the one Zavyer Uchiha placed on him, though due to the complex nature, it failed.