Byakugan

The is the  dōjutsu    kekkei genkai  of the Hyūga and Ōtsutsuki clans, inherited from Kaguya Ōtsutsuki. It is regarded as one of the, the others being the Sharingan and the Rinnegan.

Overview
This dōjutsu usually is manifested as very distinctive eyes, characterised by their enlarged and featureless white irides (though the anime tends to portray them as a light shade of lavender) with no visible pupils. While active, the pupils become more distinct and the veins near their temples bulge. Unlike other dōjutsu, the Byakugan is normally available to its users from their birth. On rarer occasions, the dōjutsu can be awakened. Himawari Uzumaki, a descendant of the Hyūga clan through her mother, generally has blue eyes. When her Byakugan is activated, she gains the characteristic white, featureless eyes of her ancestors.

In the novel adaptation, it was shown that a Hyūga descendant of a more diluted bloodline may end up only awakening this dōjutsu in a single eye.

The Byakugan's abilities have left it highly coveted, as evidenced by other villages' attempts to steal it; Kumogakure used various machinations to obtain a Byakugan during the "Hyūga Affair". Ao of Kirigakure is the only foreign power known to have obtained a Byakugan from a Hyūga he defeated and he goes to great lengths to protect it; when Danzō Shimura, a loyal extremist of Konoha, finds out about this, he goes to similar lengths to retrieve or destroy it. Unlike other dōjutsu, transplanted Byakugan can be activated and deactivated at will. In order to reduce the probability of their eyes falling into enemy hands, the main house of the Hyūga place cursed seals onto the foreheads of branch house members which, among other things, seals away the Byakugan upon their death.

Abilities
The Byakugan gives the user a near 360º diameter field of vision, with one blind spot at the back of the neck above the first thoracic vertebra. The range of this vision varies between users and can be improved with training: in Part I, Hinata Hyūga can see targets 50 metres away; by Part II, she can see targets 10 kilometres away. Its vision can penetrate through most solid objects and obstructions, though certain barriers may distort the Byakugan's perception. The Byakugan is able to follow high-speed movements, which allows the user to better react to them. In the anime, the Byakugan is shown magnifying in on small targets and seeing infrared to detect a target through their body heat. In the anime, Kaguya could use her Byakugan to see through one's thoughts and feelings, which even Hamura proved incapable of repelling with his Byakugan. She could also emit powerful chakra pulses to subdue her opponent.

Arguably, its greatest ability is to see chakra, which it can do to a greater degree than the Sharingan: it can identify chakra by the specific individuals it originates from, determine when genjutsu is being used, and discern certain types of clones from the real person. Its ability to see chakra is acute enough to see the chakra pathway system and the 361 tenketsu that run along it. The Hyūga have devised a style of combat known as the Gentle Fist to take advantage of this ability, wherein they strike an opponent's tenketsu to either seal or forcibly open them, thus giving the user complete control over their opponent's chakra.

According to Hagoromo, the Byakugan can also be used to extend the range of genjutsu. Kaguya did so in tandem with the Rinne Sharingan to cast the Infinite Tsukuyomi on whole nations.

Prolonged use of the Byakugan causes severe but temporary eye-strain, leaving wielders unable to properly perceive targets.

Evolution
When the Byakugan of a Hyūga clan member is combined with the chakra of an Ōtsutsuki clan member descended from Hamura Ōtsutsuki, over a certain period of time it evolves into the Tenseigan. When Toneri Ōtsutsuki gains the Tenseigan by taking the eyes of Hanabi Hyūga, he accesses the Tenseigan Chakra Mode, granting him Truth-Seeking Balls and physical enhancements similar to the Nine-Tails Chakra Mode. The Tenseigan also provides the ability to control gravitational forces and other telekinetic-like abilities. The Tenseigan can lose its power and revert to the Byakugan if weakened enough in battle.

Trivia

 * The kanji for can also be read as "shirome" or "hakugan". When the latter two readings are used, "白眼" is used to refer to the sclerae.
 * In the anime, the Byakugan's sight is normally illustrated in an inverted black and white colour scheme, however, instances in the manga show no such changes in their sight.
 * Boruto and Himawari Uzumaki, despite being children of Hinata Hyūga, do not have the characteristic white eyes inherited by Hyūga descendants. According to Masashi Kishimoto, he simply forgot to give it to them.
 * Kishimoto later corrected his mistake for Himawari by giving her the Byakugan in the Zai no Sho's special chapter The Day Naruto Became Hokage. While activated, her eyes take on the normal characteristics of the dōjutsu. When not in use, she has her original blue eyes.
 * The Hyūga sometimes use hand seals when activating their Byakugan, though this is not done consistently. The used seals are: Horse, Tiger with lifting just index fingers, Boar with nearly straightened fingers, Hare, Rat, Dog, Horse, Dog, Hare, Rat, Boar with nearly straightened fingers, and Snake with the right index finger lifted straight up. Sometimes, only the last seal is used.
 * Kishimoto stated in the third databook that if a Sharingan user and a Byakugan user were to mate, their offspring would have one Byakugan and one Sharingan.
 * In Naruto Shippūden the Movie, the Byakugan is able to see the elemental affinity of others.