Rasengan

The Rasengan is a spinning ball of chakra formed and held in the palm of the user's hand, which is only known to a handful of shinobi.

Overview
The Rasengan was created by Hashrama Senju, which he based on the Tailed Beast Ball. He spent three years creating the Rasengan, which he intended to be the highest form of shape transformation. Like many of Minato's jutsu, the Rasengan does not require hand seals to perform. Once it is formed, it also does not require any additional chakra to sustain it, which means that there is no definite limit to how long the Rasengan can be maintained. However, the Rasengan will dissipate if it collides with something, even if it's an unintended target. Though the Rasengan's shape is consistent from user to user, its size varies from only barely large enough to be noticeable to much larger than the user's hand. With greater experience with the Rasengan comes a larger size.

Jiraiya describes the Rasengan as stronger than the Chidori, a statement that Sasuke Uchiha concurs with after comparing the damage caused by the two attacks. On contact, the Rasengan grinds into its target, propels them along the Rasengan's trajectory - either away from the user or into the ground beneath them - and at some point bursts, causing severe damage at the point of contact.

Learning
The Rasengan requires extremely refined chakra control, something that very few ninja naturally possess. For this reason, the Rasengan is difficult to master. In order to help others gain the necessary chakra control, learning to use the Rasengan is broken into three steps:
 * 1) Rotation - the user learns to spin their chakra in multiple directions at once. To help in this regard, users may be given a water balloon so that they can mark their progress by the churning water; this step is complete once the users is able to burst the balloon with their chakra. It is important for the user to know which direction their body naturally spins its chakra for this step.
 * 2) Power - the user must increase 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.
 * 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.

Completion of the final step is by far the most difficult. For this reason, Naruto Uzumaki and Konohamaru Sarutobi both use shadow clones to help them with forming the Rasengan, the clone's hands being used to create the shell. This gives each of them a shortcut to using the Rasengan without spending the time to fully master their chakra. Both do eventually learn to use the Rasengan without clones: Naruto during the Fourth Shinobi World War; Konohamaru during the decade after the war.

Improvement
There are two broad methods by which the standard Rasengan's offensive potential can be increased: making the Rasengan itself bigger (as with the Big Ball Rasengan) or by using more than one Rasengan at a time (as with Spiralling Serial Spheres). Having absorbed Konohamaru's Rasengan, Momoshiki seamlessly melded it with his chakra to drastically increase its size.

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 initially 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, thereby creating the Wind Release: Rasengan. By further shaping the Wind Release: Rasengan, Naruto creates the Wind Release: Rasenshuriken. Boruto Uzumaki unknowingly adds his lightning-nature to his Rasengan, allowing him to throw it at targets.

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. While in Sage Mode, users have an easier time forming larger Rasengan, such as the Ultra-Big Ball Rasengan. For Naruto, it is while using Sage Mode that he is first able to throw the Wind Release: Rasenshuriken, which he was previously unable to do because it would sporadically lose its shape.

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, as seen with the Sage Art: Magnet Release Rasengan.

Trivia

 * The colouring of the standard Rasengan varies throughout the manga. On the cover of Volume 52, it's yellow, while in Boruto: Naruto Next Generations and on the covers of Volume 67 and third art book it is blue.
 * In the anime, when Naruto and his shadow clones used the Rasengan underwater, the rotation force was capable of creating a whirlpool.
 * In the anime, Naruto was capable of throwing his Rasengan, but in the manga, it was established that the Rasengan cannot be thrown.
 * In Naruto: Ultimate Ninja 3, Jiraiya was capable of spewing fire onto his Rasengan, creating the Fire Release: Great Flame Rasengan.
 * According to the Fourth Databook, Minato tried to either combine Fire, Wind, or Lightning Release to his Rasengan before his untimely death.
 * Katasuke was able to use this technique through his Shinobi Gauntlet.