Byakugan

The is a dōjutsu kekkei genkai of the Hyūga clan. Those that inherit the blood of this clan have almost featureless white eyes (Neji's and Hinata's eyes seem to have tints of lavender and blue-gray, respectively). When the Byakugan is activated, the user's pupils become more distinct, and the veins near their temples bulge. It is assumed that all members of the Hyūga clan possess the Byakugan. It is one of the three great dōjutsu, along with the Sharingan and the Rinnegan.

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 Byakugan from an opponent he defeated, and goes to great lengths to protect it; at the same time, Danzō Shimura goes to equal lengths to retrieve or destroy it. Unlike transplanted Sharingan, Ao's Byakugan can be activated and deactivated at will.

Abilities
The first 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.

The second ability is being able to see through solid objects, and can use a 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 meters, but later he increases his visual range to at least 800 meters, 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, as well as the chakra circulation system inside another's body. It is from this particular ability that they 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 defense against the Gentle Fist, since one cannot train one's internal organs to become stronger. The only possible defense is to stop the Gentle Fist's hits from connecting.

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. 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 is the greatest taijutsu style known in all of Konohagakure.

Trivia

 * In the manga, while Hinata is fighting the Deva Path, she is seen with only one Byakugan active. It is unclear if this is a unique ability or a mistake, as both Byakugan are active in subsequent panels. However this might be a result of her hair covering the characteristic vein bulging. This is not shown in the anime.
 * Sometimes the users of Byakugan make some hand seals when activating the ability, though it is also able to activate without any seals. The used seals are: Horse, Tiger, Boar with nearly straightened fingers, Hare, Rat, Dog, Horse, Dog, Hare, Tiger with lifting just index fingers, Rat, Boar with nearly straightened fingers, Dog and Snake with the right index finger lifted straightly. Sometimes just the last seal is used, and most commonly no seals are used.
 * It has been rumored that the Sharingan was somehow derived from the Hyūga clan's Byakugan.