Sharingan

The Sharingan (写輪眼, Sharingan; Literally meaning "Copy Wheel Eye", Meaning (Viz) "Mirror Wheel Eye") is a bloodline limit, or Kekkei Genkai, which appears in members of the Uchiha clan of Konohagakure. Some think that it was derived from the Byakugan many years ago. It allows the user to comprehend the attack of an opponent almost before the attacker knows what attack they are making. The Sharingan also allows the user to see the flow of chakra within a person, but to a lesser extent than the Byakugan.

Stages
When the Sharingan is first activated by a user, it has a different number of tomoe around a central pupil. The wielder of the Sharingan, through continued use, evolves the Sharingan which involves gaining tomoe. The maximum number of tomoe in a conventional Sharingan is three. When the user gets three tomoe, he becomes able to predict all movement from his opponent. The only level up from there is the Mangekyo Sharingan, which is said to be gained by seeing the death of someone close to them.

Abilities
The first of the Sharingan's powers is being able to see the chakra flow. 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 perfect accuracy, allowing the user to use the techniques as his own, or even modify them to create his 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 simple eye contact. When used against animals, the eyes of the target take on the pattern of the users Sharingan while under its influence. When used in conjunction with the other aspects of the Sharingan, this allows the user to perfectly mimic every move their opponents make even before they do, making it appear as if the user can "see the future". More advanced Sharingan users can take the hypnosis ability even further by tricking or even trapping their opponent within an illusion. 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.

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.

It has also been revealed that the Sharingan has the power to tame and control Bijū and even some summoned animals, the most noticeable version of this was used by Madara to control the Kyūbi.

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 has a Sharingan on his right side, that he gained from Shisui Uchiha which he keeps hidden under his bandages. Danzō also possesses ten Sharingan in his right arm.

Transplanting the Sharingan from a living or deceased Uchiha is the only way to avoid the loss of sight over time from the repeated use of the Mangekyō Sharingan. The user is then said to have an "Eternal" Mangekyō Sharingan.

Mangekyō Sharingan
The Mangekyō Sharingan (万華鏡写輪眼, Mangekyō Sharingan; Literally meaning "Kaleidoscope Copy Wheel Eye") is a heightened form of Sharingan. The Mangekyo Sharingan is distinguished from a normal Sharingan through its appearance, which changes the form of the tomoe seal. The appearance differs from user to user; Kakashi's and Itachi's both resemble pinwheels, while Madara's and his brother's more look like more detailed forms of the basic three-tomoe Sharingan.