By establishing eye contact with a target, the Sharingan user locks them within a genjutsu for one of various purposes: instantaneous, but temporary, unconsciousness in the target; forceful extraction of any information the target possesses; relaying memories to the target; removing genjutsu placed on the target by others; controlling a target's actions. Members of the Uchiha clan are particularly famous for using their Sharingan to control the Nine-Tails, such that Konoha quickly came to suspect an Uchiha's involvement in the Nine-Tailed Demon Fox's Attack.Sasuke Uchiha is noted to be able to use the technique through his Rinnegan, to trap tailed beasts in a genjutsu with a mere glance.
The illusions created by Genjutsu: Sharingan do not appear to have any trademark form, appearing wildly different between the same functions and even the same users. For example, Sasuke typically uses it to render targets unconscious, yet on the first occasion the target's body is pierced with large feathers, on the second the target is grasped and overwhelmed by a shadowy figure, and on the third the target experiences Sasuke personally stabbing them.
Sasuke and Itachi Uchiha have both been shown activating their Mangekyō Sharingan to cast genjutsu despite already having their regular Sharingan activated, implying that this technique is affected in some way by the evolved dōjutsu, or that there is a similar technique utilised only by the Mangekyō Sharingan.
A shadowy version of Sasuke's humanoid Susanoo first appears in a genjutsu he creates, only making a formal début several chapters later.
By making eye contact with a jinchūriki, Obito could use this technique to control the tailed beast sealed inside them, while its host remained unaffected.
Despite genjutsu working by altering the chakra flow of a target's cerebral nervous system, this technique affected Mecha-Naruto who doesn't have a brain.
Madara is shown having to deactivate his Rinnegan to his Mangekyō Sharingan to perform this technique. However, this wasn't required in Sasuke's usage.