Hunter-nin Corps (追い忍部隊, Oinin Butai), also known as the Corpse Processing Team (死体処理班, Shitai Shori Han) is a special team within Kirigakure's Anbu, consisting of hunter-nin (追い忍, oinin, English TV: tracker ninja), which are given the specific duty of hunting down missing-nin. They completely erase the person they're searching for by means of their superior assassination techniques and medical skills.
They could be seen wearing standard armoured Anbu uniform or, in some instances (such as Haku and Ao), a blue kimono with a striped, turtle-necked undershirt. They may also wear a mask with thin slitted eye-hole and a wavy design in place of the mouth, and their village symbol on the forehead. Their masks have human resemblances, rather than the typical animal resemblance most Anbu masks possess. They kill their target, taking the head as proof, and then completely dispose of the body by using various methods, such as cremation or the summoning of wild carrion crows to devour the corpse. This is done in order to make sure the village secrets which a ninja's body holds — such as chakra types, herbs eaten, ninjutsu, special medicine, and kekkei genkai — will not be revealed to outsiders.
Hunter-nin are supposed to destroy the corpse at the very spot the body is lying; it is not to be moved after it is dead, and only the head is taken back to prove that the missing-nin is in fact dead. As a requirement, they must possess a thorough and intimate knowledge of human anatomy. Due to their reputation for utterly and completely obliterating the remains of their targets, they are code-named "The Undertaker Squad" (当る隊, Atarutai).
Early in the series, Haku poses as a hunter-nin as a ruse to rescue the supposedly killed Zabuza Momochi. Ao, as a true hunter-nin himself, demonstrated great skill in tracking, as his targets grew nervous enough to lay traps to stop him. In the anime, a squad of hunter-nin led by Tsurugi was sent to hunt down the rogue jinchūriki Utakata.
In the anime, the team's background is further expanded, with some of its members (one of whom is seen sporting a mask and a hairstyle identical to Haku's, suggesting they might be typical of the group) being revealed wearing flak jackets and using special techniques, such as wielding water ropes to restrain enemies, which can be electrified in order to harm victims, and creating mist which sucks chakra out of the opponents.
Hunter-nin also possess a barrier ninjutsu to protect certain body parts, such as an eye. This technique is of the utmost importance and is so secretive that no one besides members of hunter-nin know how to undo it, not even the Mizukage.