Rasengan

The Rasengan is a spinning ball of chakra, invented by Minato Namikaze.

Overview
Minato came up with the idea for this technique after seeing a Tailed Beast Ball being used. Because his lover, Kushina Uzumaki, was a jinchūriki, she would inevitably be deployed to fight opponents who would use Tailed Beast Balls against her. Minato therefore wanted to develop a technique that Kushina could use to match against those opponents, which also worked with her chakra volume as a jinchūriki. Minato struggled with the original concepts for the technique, as they were either too weak or too unstable for use. It was only from a conversation with Jiraiya that Minato thought of a way to make the technique usable. Because of Jiraiya's inspiration, Minato wanted to call the technique, but both Jiraiya and Kushina felt it was too long and lame. Kushina suggested "Rasengan" instead, which Minato agreed was a better name. Minato would continue refining the Rasengan, ultimately taking three years of development time to perfect it.

To perform the Rasengan, users gather chakra in the palm of their hand, which they spin rapidly in multiple directions in order to contain the chakra into a spherical shape. No hand seals are involved, in theory making it easy to learn and use, nor does it require additional chakra to maintain once it's properly formed. The Rasengan grinds into whatever it comes into contact with, doing extensive damage until the spinning chakra dissipates; Jiraiya describes it as more powerful than the Chidori, an assessment that Sasuke Uchiha agrees with after comparing the two. If users thrust the Rasengan into targets, they can be propelled away from the user until they eventually collide with some obstacle. The Rasengan's various attributes have earned it recognition as the pinnacle of shape transformation.

The Rasengan requires a great deal of chakra control to perform, making it incredibly difficult to learn. For this reason, Jiraiya breaks up the process into three steps when he teaches it to Naruto Uzumaki. Naruto subsequently uses these same three steps when he teaches it to Konohamaru Sarutobi, who in turn later uses them when he teaches it to Boruto Uzumaki:
 * 1) Rotation - the user learns to spin their chakra in multiple directions at the same time. Users may be given a water balloon while they practice, to help them mark their progress by the churning water. The step is completed when users are able to burst the water balloon with their chakra. In theory this should be done with one hand, but Naruto initially uses chakra from both of his hands to increase the agitation, thus saving time. It can be useful to know in which direction one's chakra naturally spins, in order to avoid impeding oneself.
 * 2) Power - the user increases the volume and density of chakra they output. To help in this regard, users may be given a rubber ball: there is no water inside to help them burst the exterior and the rubber shell is thicker than a balloon's. It's important that users are able to actually destroy the rubber ball, not merely rupture it, as only by destroying the ball will users know that their chakra is sufficiently concentrated. This step can be painful for users, as they may not be accustomed to emitting so much chakra from their hands at once, and in fact they may end up injuring themselves in the process of destroying the ball.
 * 3) Containment - the user must combine the first two steps into a contained sphere. To help in this regard, users may be given a balloon to help them visualise the intended shape; if the balloon is popped or otherwise moves, mastery is not yet achieved. This is by far the most difficult of the three steps because of how refined the user's chakra control must be. Naruto and Konohamaru end up using shadow clones as a shortcut, the clone's hands helping them keep the chakra contained. Both do eventually learn to perform the Rasengan without the need for shadow clones, Naruto during the Fourth Shinobi World War, Konohamaru by the time he begins leading Team 7.

Once learned, the Rasengan appears largely identical from one user to the next. The only potential difference is its size: when Boruto first learns to use the Rasengan, it's noted to be quite small. With greater proficiency, the size of a user's Rasengan grows to be about the size of their hand.

Improvement
Because of how useful the Rasengan is once it's been learned, many users end up inventing variants or additional applications based upon it. Creating larger versions of the Rasengan is one of the more straightforward methods, as seen with its Big, Ultra-Big, and Super-Ultra-Big versions. Another option is to increase the number of Rasengan that the user creates: most users are limited to two (one in each hand), but Naruto also develops a practice of having his shadow clones perform their own Rasengan, giving him dozens of Rasengan to attack with simultaneously. In the anime, Boruto devises a way of compressing the Rasengan, thus increasing its penetrative strength.

Minato created the Rasengan with the intention of combining it with his own nature, a task that proved too difficult for him to complete prior to his death. Kakashi Hatake also failed to combine the Rasengan with his own lightning-nature, causing him to invent the Chidori instead. Naruto encounters similar struggles to combine the Rasengan with his wind-nature, which he describes as needing to look left and right at the same time. He is eventually able to overcome this obstacle by using a shadow clone to provide the nature transformation while he and another shadow clone form the Rasengan. This ultimately leads to the creation of the Wind Release: Rasenshuriken, which is most notable for its ability to be thrown. Years later, Boruto unknowingly adds his own lightning-nature to his Rasengan, which causes it to turn invisible if thrown, potentially lowering an opponent's guard before it strikes them.

Senjutsu Influence
When in Sage Mode, the user can use a senjutsu-enhanced version known as. It is fundamentally the same as the normal version, except stronger. For Naruto, it is while using Sage Mode that he is first able to throw the Wind Release: Rasenshuriken, which previously would sporadically lose its shape if he didn't hold on to it.

Tailed Beast Influence
Naruto, as a jinchūriki, can use the Nine-Tails' chakra to assist in using the Rasengan. While using a version 1 cloak in Part I, he makes a Rasengan without the aid of a shadow clone, using the cloak itself to contain it. Due to the Nine-Tails' influence, this Rasengan is darker in colour and much stronger. Different video games have called it, , , and. After learning to use the Nine-Tails Chakra Mode in Part II, Naruto uses chakra arms in place of a shadow clone to help him create the Rasengan. Because he can manifest multiple chakra arms at once, this in turn means he can use multiple Rasengan simultaneously, such as for the Spiralling Strife Spheres.

The Nine-Tails is uncooperative when Naruto first starts using the Nine-Tails Chakra Mode, preventing him from using the Tailed Beast Ball. He tries to create an alternative in the Tailed Beast Rasengan, but he's unable to do so. The Nine-Tails does eventually agree to work with him, allowing him to use Tailed Beast Ball and thus making the Tailed Beast Rasengan unnecessary. Naruto is later given access to the other tailed beasts' chakra, whose natures he can apply to the Rasengan and its variants, such as the Sage Art: Magnet Release Rasengan.

Trivia

 * The colouring of the standard Rasengan varies throughout the manga. For example, it's yellow on the cover of volume 52, blue on the cover of volume 67, and orange on the cover of Weekly Shōnen Jump #35 (2005).
 * The anime is more consistent about depicting the Rasengan as blue, with the exception of The Last: Naruto the Movie, which depicts it as orange.
 * While Minato intended to pass this technique on to Kushina, it is unknown if he succeeded.
 * In the anime, if used underwater, the rotation force of the Rasengan is capable of creating a whirlpool.
 * The anime shows Naruto being able to throw the Rasengan, something that is later established to require special training.
 * Katasuke uses this technique through his Shinobi Gauntlet.
 * In Naruto: Ultimate Ninja 3, Jiraiya spews fire onto his Rasengan, creating the Fire Release: Great Flame Rasengan.
 * Jump Super Stars is one of the few video games where Part I Naruto is able to access his non-clone-refined version of his Rasengan, dubbed as.