Sharingan

The Sharingan 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 the user, it has one to two 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. The only level up from there is the Mangekyo Sharingan, which is said to be gained by killing one's best friend. This bit of information must be taken with a grain of salt however, because Hatake Kakashi developed the Mangekyo Sharingan during the two-and-a-half year period of time when Naruto was being trained by Jiraiya. Four people are currently known to have the Sharingan in any form, they are Uchiha Sasuke, Uchiha Itachi, Tobi Who said his power is the power of Uchiha Madara, and Hatake Kakashi, who obtained his Sharingan from Uchiha Obito, one of his original teammates.

Origins
The Sharingan (写輪眼, Sharingan, literally "Copy Wheel Eye", Viz "Mirror Wheel Eye") is a special type of pupil condition that naturally occurs in all members of the Uchiha clan, though not all of them gain the ability to use it. Itachi hints at some dark ultimate purpose behind the Sharingan, which both he and Sasuke apparently know, but the series has yet to reveal exactly what that purpose is. The Sharingan is thought to be descended from the Byakugan, but grants the user different abilities and a different insight.

Abilities
The Sharingan's first and most well known ability is to memorize any technique that it witnesses. It can memorize ninjutsu, genjutsu, and taijutsu with perfect accuracy, allowing the user to use the techniques as his own. In order to reproduce 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 hard as Lee has. In addition to physical skill, the Sharingan cannot allow the user to reproduce other kekkei genkai since the Sharingan user doesn't possess the same genetic trait. Additionally, Sharingan users cannot summon a creature which they have not made a contract with.

The Sharingan also grants the user incredible clarity of perception, allowing them to track fast-moving objects with ease and even predict the subsequent movements of those objects. The ability to track and predict movement is largely based on individual skill, as unskilled users have much more trouble tracking fast-moving objects. A fully-evolved Sharingan eye can track all but the quickest opponents, even if the user can't move at the same speed. This perception is also great enough to see through genjutsu, depending upon the skill of the user, and the ability to observe the movement and flow of chakra, but not to the same extent or clarity as the Byakugan. Additionally, the Sharingan grants the user exceptional analytical powers, allowing them to instinctively pick up on various visual clues with little difficulty. In one instance, Kakashi had to shut his eyes to avoid accidentally reading Naruto's lip movements while he was spoiling the ending of one of Kakashi's Icha Icha books. Sasuke is also able to reproduce answers on another person's test by watching their pencil movements during the Chunin Exam.

Finally, the Sharingan is capable of a unique brand of hypnosis that involves suggesting actions and thoughts to the opponent. When used in conjunction with the other aspects of the Sharingan, this allows the user to perfectly mimic every move their opponent makes, even before they do, making it appear as if the user can see the future.

Receiving the Sharingan
The Sharingan does not automatically appear from birth in a user; instead, it will usually appear when the user is in a life-threatening situation, where the power of the Sharingan could save the user. After this first appearance, the user can then call up the Sharingan any time he or she pleases. (Due to its unique nature, Kakashi's Sharingan cannot be deactivated, and thus he usually covers it to conserve chakra.) The power of the Sharingan is judged through the number of tomoe in the eye, known as the "tomoe seal." It is not unheard of for a Sharingan user to have different amounts of tomoe in each eye. For example, Sasuke's Sharingan initially had two tomoe in one eye and one in the other. Upon his first regular activation of it, it had evolved to two tomoe in each eye. He evolved it to three in each eye during his battle with Naruto at the Valley of the End. A fully-matured Sharingan has three tomoe in each eye.

Transplanting the Sharingan
It is possible to transplant Sharingan eyes from one person to another, as occurred with Kakashi and his deceased comrade Obito Uchiha. However, transplanted eyes do not work as well in a non-Uchiha body, causing more strain on the user, as is the case with Kakashi – extended use of his Sharingan will cause him to collapse. Uchiha clan members are not burdened with this weakness; while activation of the Sharingan does consume chakra, the drain is so small as to be almost insignificant. Itachi in particular seems to have no trouble keeping his Sharingan activated near-constantly.

Characters who possess the Sharingan
Characters who currently possess the Sharingan include Sasuke Uchiha, Itachi Uchiha, and Kakashi Hatake. Kakashi's eye was given to him by Obito Uchiha, one of his teammates in his original three-man ninja team. Obito developed it in a battle against a camouflaging Rock shinobi.

Mangekyou Sharingan
The Mangekyo Sharingan (万華鏡写輪眼, Mangekyō Sharingan, literally "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. However, unlike the Sharingan, it may have a different appearance between the users, as Kakashi's version is significantly different than Itachi's. Those who have the potential to obtain it must make a great sacrifice in order to obtain it – according to Itachi, they must kill their closest friend. Itachi does in fact do this to obtain his version, but Kakashi has noted that everyone he cared about had died long before the start of the series, so how he developed his own variant is unknown. It has also been implied in the manga that use of the Mangekyo Sharingan may deteriorate the user's normal eyesight.

In the long history of the Uchiha clan, only a few have been able to obtain the Mangekyo Sharingan. At present, there are only two people who can use the Mangekyo Sharingan, Itachi being one and Kakashi being the other. According to Itachi, if his younger brother Sasuke obtained it, the number of people who could handle the Mangekyo Sharingan (including himself) would be three. Itachi's statement may suggest that there is (or was at that time) another Mangekyo Sharingan user somewhere; however, Itachi uses the verb atsukau (扱う) which means "to handle" or "to deal with", which could also mean that there are only three people left who are capable of acquiring the Mangekyo Sharingan. Given that Itachi had just killed every other member of the Uchiha clan, this statement could simply be taken as a matter of fact: Kakashi, Sasuke, and Itachi were the only known characters left alive with Sharingan eyes, so no one else had the potential to obtain it.