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; relay memories to the target; remove 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-Tails' Attack on Konoha.Sasuke Uchiha is noted to be able to use 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, when already having their regular Sharingan activated, implying that the technique is effected in some way by the evolved dōjutsu, or that it's 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 genjustu working by altering the chakra flow of a target's cerebral nervous system, this technique effected Mecha-Naruto who doesn't have a brain.
Madara is shown reverting his Rinnegan to his Mangekyō Sharingan to use this technique. However, Sasuke's Rinnegan, having a secondary stage, was shown to be able to cast a more powerful version of this technique.