Byakugan

The is the dōjutsu kekkei genkai of the Hyūga clan. It is one of the, along with the Sharingan and the Rinnegan.

Background
Those who inherit the blood of this clan have almost featureless white eyes. When the Byakugan is activated, the user's pupils become more distinct, and the veins near their temples bulge. It also appears that unlike the other two great dōjutsu, all members of the clan possess and can use the kekkei genkai from birth, as opposed to needing to awaken or not inheriting it at all.

The Byakugan's abilities have left it highly coveted by the other villages, as evidenced by Kumogakure's attempt to steal it during the Hyūga Affair. Ao of Kirigakure was able to obtain a single Byakugan from a Hyūga he defeated, and goes to great lengths to protect it. At the same time, Danzō Shimura went to equal lengths to retrieve or destroy it. Unlike a transplanted Sharingan, a transplanted Byakugan can be activated and deactivated at will.

Abilities
The primary ability of the Byakugan gives the user a near 360º field of vision, except for one blind spot at the back of the neck above the third thoracic vertebra. Upon activation of the dōjutsu, the wielder is granted an automatic 50 metre field of vision around themselves which can then be extended at the user's discretion.

The second ability is being able to see through solid objects, as well as some degree of telescopic sight. The distance and the degree of telescopic sight varies from person to person, and can be improved. In Part I, Neji Hyūga was able to see an area of 50 metres, but he later increases his visual range to at least 800 metres, while Hinata Hyūga's range in Part II stretches at least as far as 10 km. However, the Byakugan cannot see through certain barriers, such as the Four Black Fog Battle Formation used by the Sound Four.

The third and one of the greatest uses of the Byakugan is the ability to see chakra, chakra flow, the chakra circulation system inside the body with great detail as well as the colour of chakra. It is from this particular ability that the Hyūga Clan are able to employ their ultimate attack style: Gentle Fist. Unlike the Strong Fist, which is focused on external injuries, the Gentle Fist style focuses on internal injuries. Each strike, when placed correctly, strikes along the chakra pathway system, which is the network of vessels that carry chakra. This network is closely intertwined with the functioning of the body's organs, and so, when the keirakukei take damage, the organs suffer as well. There is no defence against the Gentle Fist, since one cannot train one's internal organs to become stronger. The only possible defence is to stop the Gentle Fist's hits from connecting. Like the Sharingan and Rinnegan, the Byakugan's ability to see chakra and its flow allows the user to detect chakra sealed barriers as well as determine when genjutsu is being used. It was shown in the anime that the Byakugan can also discern a transformation and a real person using the eye's ability to see chakra.

Particularly skilled users of the Byakugan can take the Gentle Fist style one step further. Along the keirakurei are 361 pinprick-sized nodes called tenketsu, or chakra points, which only the more skilled users can see. When struck, the Byakugan user can choose to either seal, or forcibly open these tenketsu, thus giving the user complete control over their opponent's chakra system. So, with precision, all those points along the victim's body can be sealed, stopping the chakra flow to those areas. Able to strike without force, and yet do more damage to a ninja than some of the most powerful ninjutsu, the Gentle Fist has been labelled as the greatest taijutsu style known in all of Konohagakure. In the Search for the Bikōchū Arc, Hinata demonstrated two other abilities with her Byakugan: the ability to magnify and zoom, effectively seeing every small insect in the area and to see the infrared spectrum allowing her to detect things through their body heat.

Constantly using the Byakugan over a long period of time causes severe eye strain in the user as seen with Neji Hyūga, who at that point, could not tell the difference between Kiba and Akamaru. This however, is not permanent.

Trivia

 * Sometimes the users of the Byakugan use hand seals when activating the ability, though it is also able to activate without any seals. 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 just the last seal is used, and most commonly, no seals are used.
 * It has been said that the Sharingan was somehow derived from the Hyūga clan's Byakugan.
 * The Byakugan is said to have the greatest vision and insights amongst all dōjutsu.
 * When depicted in the anime, the eyes of the Hyūga tend to have slight hints of colour in them, despite the fact that they are supposed to be completely white.
 * In the anime, the Byakugan's sight is normally illustrated in black and white. However, there are instances in the manga that contradict this.