Kunai

Kunai, along with the shuriken, is one of the most common ninja tools in the Naruto series. It is a black dagger designed for thrusting and stabbing, though it can still do some damage if thrown despite not being designed for it, and is about the length of one's hand.

Many shinobi attack by attaching an exploding tag to the blade. The kunai is usually thrown so that it lodges in an object near the target, and then the explosion from the attached tag destroys the target. Kunai can also be kept on spring-loaded wires up the user's sleeve, as seen in Naruto Uzumaki's fight with Sai when they first met. They can be used for close combat, like a normal knife. Empowered with "chakra flow", kunai thrown into targets can cut through solid wood and even solid stone targets.

Shinobi appear to favour them so much due to both their sturdy and dependable nature, as well as their evident versatility.

Types of Kunai
Different types of kunai have been seen in the series:
 * Zabuza Momochi was shown using a special kunai that curved inward on one side, making it more suitable for stabbing and thrusting. This type is also shown used by Kirigakure in the anime.
 * Kabuto Yakushi, when threatening to kill a hospitalised Sasuke Uchiha in Part I, held a kunai with a curved blade to Sasuke's neck (seems to be the same type that Rōshi used).
 * Before the first part of the Chūnin Exams, Zaku Abumi used a kunai with holes in the middle and no ring at the end.
 * During the first part of the Chūnin Exams Kotetsu Hagane threw a kunai at Sukima, that had a tassel connected to the ring.
 * During the second part of the Chūnin Exams, Team Oboro used double-bladed kunai while facing Team 7.
 * Shiin used kunai with long, thin, red ropes attached to them.
 * In the anime, a young Naruto finds a special Takigakure kunai with an extending blade on one side.
 * Minato Namikaze used three-bladed kunai that had a special seal applied to them in order to perform his Flying Thunder God Technique.
 * The Father puppet, created by Sasori, was equipped with a segmented wooden whip covered with kunai.
 * During their battle with Hidan, Izumo Kamizuki and Kotetsu used kunai the size of ordinary blades to attack him. When next observed, the pair were connected by a length of chain, with a weight attached to its centre.
 * In the manga, the Prajñā Group used very long kunai to assault Danzō Shimura. However, in the anime, these were replaced instead by the standard variant of kunai and later, katana.
 * An unnamed Iwa shinobi of the First Division used a kunai with a manriki-gusari attached to its hilt, in a similar manner to a kusarigama during their initial engagement with the Akatsuki forces in the Fourth Shinobi World War.
 * An unnamed Kiri shinobi of the Fourth Division used a kunai with a noticeably serrated edge along at least one of its sides, during their first direct engagement with the Second Mizukage and Third Raikage in the Fourth Shinobi World War.
 * Kusuna used a double-bladed kunai that can disconnect into two kunai during his first attempt to assassinate Shion, and the kunai each have a grip around the handle.
 * Deidara was shown wielding a kunai with a square hilt in his mouth.
 * Tenten used a smaller version of kunai when threatning Menma.
 * Among the many weapons created by Iō there were two variations of a kunai: a double-bladed one without a handle and poison on the blade (and therefore, dangerous to the user) and one with another handle instead of a blade. Obviously, they were basically useless.

Kunai Launching Weapons

 * Tenten used a grenade that releases kunai and shrapnel in an omnidirectional manner once it detonates.
 * Chiyo used a simple length of cord to thread numerous kunai together, in such a manner that the entirety could be released and thrown consecutively with just a single motion. A similar arrangement was employed by a shinobi of the Third Division, during their attack on the Seven Ninja Swordsmen of the Mist, in the Fourth Shinobi World War.
 * Dotō Kazahana's army of Yukigakure ninja used mortars mounted on their train compartments to launch a massive amounts of kunai at a very rapid rate.
 * The ninja of the Land of the Sky equipped themselves with hand-held launchers that can fire kunai rapidly, similar to machine guns.

Trivia

 * The alternative spelling for "kunai" is written with the characters 苦無, which translate as "no hard feelings" ("bitter", "hardship" and/or "suffering" (苦), and "negative", "no", "not", "lack" and/or "have no" (無)).