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.
When the Sharingan is first activated by the user, it has one 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 killing someone close to them. Kakashi, however, seems to have developed the Mangekyo Sharingan on his own.
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 physically 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, (so even if the user can follow his or her opponents movements it doesn't necessarily mean their body will be able to react fast enough even though they saw the attack coming). 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. Another ability of the Sharingan is the ability to see chakra as color, even through other objects. For example when Sasuke fights Deidara, he can see Deidara's bombs hidden under ground because of their chakra. 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 was also able to reproduce answers on another person's test by watching their pencil movements during the Chunin Exam.

The Sharingan also is capable of a unique brand of hypnosis that involves suggesting actions and thoughts to the opponent through simple eye contact. 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. 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 Genjutsus.

Receiving the Sharingan:
Unlike the Byakugan, 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, however, 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 insignificant. Itachi in particular seems to have no trouble keeping his Sharingan activated near-constantly.

Transplanting the Sharingan eyes from a living or deceased Uchiha is the only way to avoid the loss of sight over time from the Mangekyo Sharingan.

Characters who possess the Sharingan:
Characters who currently possess the Sharingan include Sasuke Uchiha, Kakashi Hatake, and Madara Uchiha. 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.