Barrier Ninjutsu (結界忍術, Kekkai Ninjutsu; Literally meaning "Barrier Ninja Techniques") is the incorporation of barriers into techniques. Essentially these techniques allow the users to manipulate barriers. The user can erect barriers that can be used to protect themselves, or to trap an opponent, amongst other various uses.
There are also several species of animals that make use of barrier techniques such as the Gourd Toad of Mount Myōboku which tiles the Barrier: Toad Gourd Prison. Barriers have also been incorporated into the form of tags for both offensive and defensive purposes.
Barriers can be used for a variety of purposes, such as to prevent outsiders from entering, as with the Five-Seal Barrier; to prevent those inside from leaving the confines of the barrier, and even to prevent those trapped within from using certain techniques, as was done to stop Dan Katō from utilising the Spirit Transformation Technique by erecting the Four Violet Flames Formation around him.
The strength and efficiency of barriers seemingly depends on the type of technique used to create the barrier itself, and the ability of others to perceive, enter, or destroy the barrier. The Self-Repairing Barrier, created by a synchronised array of multiple shinobi, is supposedly able to contain a jinchūriki and its tailed beast, but has been shown to be unable to handle the combined efforts of two jinchūriki from breaking out. The Four Red Yang Formation a powerful barrier only usable by four Kage-calibre shinobi. The Six Red Yang Formation is the highest level barrier in existence, usable only by those who possesses the Six Paths power.
Some Barrier Ninjutsu seems to have a tie-in with Space–Time Ninjutsu, by manipulating the very barrier of space-time continuum itself: examples include the inside of the Toad Gourd Prison, that is a barrier of a different world, while Kamui transfers the target to the other dimension and the Flying Thunder God: Guiding Thunder warps the target to another kunai's location.
Although barriers are meant to contain an area from outsiders from entering or vice-versa, they can be breached in different manners. For certain barriers, users can willingly create openings for people to pass through, or if others know a certain hand seal to temporarily open a barrier without dispelling it, such as Deidara opening the Five-Seal Barrier or Itachi Uchiha allowed himself and Kisame Hoshigaki to slip through the Sensing Barrier.
Some barriers can be broken by sheer force, as Kakuzu punched through the Sealed Iron Wall or Obito Uchiha tore through the Four Red Yang Formation. This depends on the strength of the attack in proportion to the field.
Disregarding the strength of any barrier, even the most powerful ones, intended to prevent detection and/or intrusion by outsiders, they can be circumvented by the use of Space–Time Ninjutsu, because barriers only create confines within the dimension that they reside in, in which Space–Time Ninjutsu can bypass them by going into another dimension. Primary examples seen are when Obito's Kamui, which he used to gain access to the place where Kushina Uzumaki was giving birth to Naruto Uzumaki, or when Minato Namikaze used the Flying Thunder God Technique to teleport a large amount of people out of Obito's Six Red Yang Formation's otherwise inescapable boundaries.