Sharingan

The Sharingan is a dōjutsu kekkei genkai, which appears in some members of the Uchiha clan. It is one of the Great Three Dōjutsu, along with the Byakugan and the Rinnegan.

Stages
The Sharingan is not automatically activated in the user by birth. It is only first unlocked under some stressful or emotional condition after which the Sharingan may be used freely. When the Sharingan is first activated by a user, it has a different number of around the central pupil, varying between users. The wielder of the Sharingan, through continued use and training, changes its form further, which involves gaining more tomoe up to a maximum of three per eye. As the Sharingan develops, the user gains heightened abilities.

Abilities
The first of the Sharingan's powers is being able to see the flow of chakra. The Sharingan can tell if a person is under a genjutsu because the person's chakra flow would be irregular.

The Sharingan's second-most prominent ability grants the user an incredible clarity of perception, allowing them to easily recognize genjutsu and different forms of chakra, though not to the same extent as the Byakugan. This also allows the user to pick up on subtle details, enabling them to read lip movements or mimic something like pencil movements. As the Sharingan evolves, gaining more tomoe seals, this ability extends to being able to track fast-moving objects before finally giving some amount of predictive capabilities to the user. However, even though the user may be able to see an attack or know it is coming, their body may not have time to react.

The Sharingan's third and most well-known ability is that it grants the user the ability to memorize any technique that he or she witnesses, apart from other kekkei genkai techniques. The user can memorize ninjutsu, genjutsu, and taijutsu with near perfect accuracy, allowing the user to use the techniques as their own, or even modify them to create their own new techniques, much like Sasuke Uchiha's Lion Combo. In order to reproduce a copied jutsu, however, one must have the necessary skill or ability to perform them. A prime example is Rock Lee's brand of taijutsu. While Sasuke was able to copy some of Lee's moves, they put a much more significant strain on him since he hasn't trained nearly as much as Lee has. In addition to physical skill and kekkei genkai techniques, the Sharingan cannot reproduce jutsu dependent on other factors, such as summons that the user has not signed a contract with.

The Sharingan's final commonly-used ability is capable of inducing a unique brand of hypnosis that involves suggesting actions and thoughts to the opponent through genjutsu cast by simple eye contact. More advanced Sharingan users can take the hypnosis ability even further to the point where a powerful summon like Manda or even the Nine-Tailed Fox may be controlled. When this was used against Manda and the Nine-Tailed Fox their eyes took on the tomoe pattern of the users' Sharingan while under hypnotic influence.

The hypnotic ability coupled with the other aspects of the Sharingan, can make the user appear as if they can "see the future". First, the user must perfectly mimic every move their opponent makes and possibly their thoughts and words through careful observation. Then, they suggest an action to the opponent such as hand seals for a jutsu that the opponent and Sharingan user already know and copy that. With these things done, the user can complete the action even before the opponent does, possibly using the Sharingan's genjutsu to confuse or delay them. As with other genjutsu, genjutsu cast by the Sharingan are capable of putting an opponent to sleep, inducing paralysis, replaying memories to someone else, or creating various illusions to deceive someone. The potency of the Sharingan's illusion is dependent on the user's knowledge and skill with implementing genjutsu.

In addition to these commonly used techniques, the Sharingan allows the user to perform the genjutsu Izanagi. Using Izanagi, the user can warp reality for a short amount of time, changing reality into illusion and illusion into reality. However, this technique has been dubbed kinjutsu, because use of it causes the user to go blind in the eye that performed Izanagi.

Although the Sharingan consumes the user's chakra while active, the chakra levels that the user must exert is so insignificant that they can keep it active constantly. In fact, Itachi Uchiha was so masterful of it, he was able to keep it active for most of the time, only once deactivating it after using Tsukuyomi on Sasuke and Amaterasu to escape Jiraiya. However, when a Sharingan is transplanted to a non-Uchiha, the chakra drain is significant, leading them to cover it up when not in use, to stop the drain.

Receiving the Sharingan
As already mentioned, the Sharingan needs to be activated by the user himself or herself under a difficult, emotional stress. After its first appearance, the user can use the Sharingan any time he or she pleases. It is also possible to transplant Sharingan from one person to another, as occurred with Kakashi Hatake, who received his left eye's Sharingan from his dying comrade, Obito Uchiha. Because of this, Kakashi's Sharingan cannot be deactivated, and as using the Sharingan requires much more chakra from him than it does from an Uchiha clan member, he usually covers it with his headband to conserve chakra. Also, Kakashi needs to rest for a longer period of time after using his Sharingan extensively in battle. In a similar way, Danzō Shimura had a Sharingan instead of his normal right eye, that he gained from Shisui Uchiha, which he kept hidden under his bandages. Danzō also possessed ten Sharingan in his right arm. Samuel was never receive the Rinnegan only when Sharingan received by himself after the battle while Naruto and his allies was shocked. Samuel tried to focus his Sharingan, but Sasuke attacked. However, Samuel overwhelming powers to becoming Hyper Saiyan 2 possessed of Susano'o powers.

Mangekyō Sharingan
The Mangekyō Sharingan gives the user access to several powerful techniques, including the powerful fire jutsu Amaterasu and the almost unbreakable genjutsu Tsukuyomi. The Mangekyō Sharingan erodes the eyesight of its users, eventually causing blindness, which can be prevented by obtaining another pair of Mangekyō Sharingan from a sibling. The new Sharingan is dubbed the Eternal Mangekyō Sharingan.

Trivia

 * The Sharingan is evolved from the Uchiha Clan Ancestor's dōjutsu, which is evolved from the Rinnegan. It is unknown where or even if the Byakugan fits into this lineage.
 * Some characters believe that the Sharingan was somehow derived from the Hyūga clan's Byakugan.
 * In the anime, two of Sasori's Hundred Puppets are seen with what appears to be Sharingan eyes.