Sage Mode

Sage Mode is the result of using natural energy along with a shinobi's normal chakra to drastically empower all of their abilities.

Overview
Sage Mode is the empowered state humans enter when they learn to draw natural energy inside them and blending it with their chakra. By doing so, the human creates new senjutsu chakra that allows them to enter Sage Mode. Sage Mode enhances the combat efficiency of the user by allowing them to tap into the natural force of the world that opens new techniques to the user and allows them to power up existing ones with the new senjutsu chakra.

Sage Mode can be learned in two places, Mount Myōboku of the toads and Ryūchi Cave of the snakes. Learning Sage Mode at Mount Myōboku or Ryūchi Cave grants the user with a toad and snake style of Sage Mode respectively, which carries over the same advantages and disadvantages, only differing in application.

Training
According to Fukasaku, only those who already possess "extreme chakra levels" can utilise natural energy to invoke senjutsu. Also, the users body must be strong enough to support Sage Mode; Orochimaru learned senjutsu but was unable to use Sage Mode because his host bodies were not strong enough to support this power. Stemming from this and his experiments with Jūgo's unique ability, however, he was able to create his own brand of unique cursed seals.

The user is required to balance his/her physical and spiritual energy with the natural energy he/she gathers from nature; Gamakichi compares the process to adding an extra flavour to a chocolate and vanilla swirl ice cream. If the user puts too little natural energy into using senjutsu, the technique will not work.

In order to enter Sage Mode, the gatherer must remain absolutely still in order to be one with nature and balance the natural energy with their own physical and spiritual energies. Those learning from the toads have access to a special toad oil that allows the natural energy to enter their body, but this oil only exists in liquid form in Mount Myōboku's atmosphere. The oil evaporates if it is taken outside, thus rendering it useful only for learning purposes. If the user takes in too much natural energy, the user will start to turn into a toad and, eventually, turn to stone. The petrified remains of one-time apprentice Sages who failed to master senjutsu are spread throughout Mount Myōboku.

Getting hit by a special staff during the transformation will reverse the toad transformation by knocking all the natural energy out of the gatherer. It is dangerous to learn how to gather natural energy without the staff because there is no other known way to reverse the toad transformation. Furthermore, if the transformation is completed, not even the staff can reverse it, and the user would become a toad statue forever.

Advantages
The advantages afforded to those who learn senjutsu include:
 * The user's physical strength, speed, stamina, reflexes, perception and durability dramatically increase.
 * The user's ninjutsu, genjutsu, and taijutsu become substantially more powerful.
 * The user can manipulate the natural energy that surrounds them in this mode, turning it into an extension of their body, which increases the reach of their attacks.
 * The user gains the ability to sense chakra around them.

Disadvantages
There are a few disadvantages while using this mode as well, including:
 * If the user draws in too much natural energy, they run the risk of transforming into a toad, and then into stone. If the petrification is completed, it cannot be reversed.
 * In order to gather enough natural energy to initiate the transformation into Sage Mode, the user must remain perfectly still. Because of that, the user can be an easy target for an opponent.
 * Because the user needs to move during combat, the senjutsu chakra cannot be replenished, which means the user can't stay within this state for extended periods of time.
 * The second and third weaknesses mentioned above can be bypassed through the Sage Art: Amphibian Technique, where one Sage gathers chakra for another or as in Naruto's case, shadow clones can be used for the same purpose.
 * After Sage Mode has ended, the user appears to experience exhaustion.

Imperfect Sage Mode
Jiraiya was not able to completely balance his own chakra with natural energy, resulting in him using an imperfect form of Sage Mode. As a result of the imbalance, Jiraiya took a number of toad-like traits when using Sage Mode (such as his big hands or his skin). His appearance was altered: his nose grows bigger and gains warts, his teeth turn sharp, he gains excess hair (his goatee), and toad-like eyes. Further transformational attributes includes him taking on a toad-like stature and the lines on his face become much thicker, taking over each side of his face. Because of these traits, Jiraiya didn't like to use Sage Mode as it "displeased the ladies". Despite his lack of complete mastery, Jiraiya could still use this form effectively in battle to augment his all-around performance. He also showed the ability to transform his limbs (e.g. his hands and feet) into those of frogs so as to aid in manoeuvrability.

In order to get around the danger of remaining stationary during a battle to re-enter Sage Mode, Jiraiya would summon Shima and Fukasaku by drawing a blood contract on his face. The Two Great Sage Toads would fuse to Jiraiya's shoulders with the Sage Art: Amphibian Technique, gathering natural energy from their stationary positions on Jiraiya's shoulders for him to use. This on the other hand leaves Shima and Fukasaku vulnerable to attacks while Jiraiya is in combat.

Sage Mode
Unlike Jiraiya, Naruto Uzumaki, Minato Namikaze, Kabuto Yakushi and Hashirama Senju were able to perfectly balance the natural energy with their own chakra. This allows them to use Sage Mode in perfect balance, which allows them to retain all of their previous physical characteristics. However, those who master Sage Mode have a distinctive marker, which is a dark pigmentation around their eyes. While the actual design of the pigmentation and the user's eyes usually differs from Sage to Sage, Naruto Uzumaki and his father, Minato Namikaze, appear to bear identical such designs. Kabuto Yakushi's snake-like characteristics progressed further in Sage Mode with him growing four horns, with him stating metaphorically that he'd transcended from "snake" to "dragon".

A true sage would still suffer from the weaknesses of Sage Mode, requiring that they use Sage Art: Amphibian Technique in a battle situation so that they could stay mobile. However, Kabuto Yakushi assimilated the DNA of Jūgo to replicate his clan's ability to passively absorb natural energy, thus allowing him to continuously collect the energy even while moving, bypassing this particular weakness.

As his own workaround, Naruto has some of his shadow clones gather natural energy elsewhere while he fights. Once he has run out of his own senjutsu chakra, he reverse summons the clone and has them disperse, giving him the clone's natural energy and allowing him to enter Sage Mode again. This method limits the maximum number of shadow clones that Naruto can create to five for as long as the clones are gathering natural energy, as anything more would interfere with their focus. To make sure he has enough shadow clones for use in battle, Naruto has only two shadow clones gather senjutsu chakra, allowing him to enter Sage Mode a total of three times. Naruto has also stated that it is possible to lengthen the time he is in Sage Mode. Naruto, Hashirama and Minato's skill in senjutsu is such that they can enter it in moments. In addition, Naruto's old limit of two Rasenshuriken has been increased to three.

In this mode Hashirama's abilities were drastically increased. This was witnessed through the use of his Sage Art: Wood Release: True Several Thousand Hands technique which created a gargantuan multi-handed structure which easily dwarfed several mountain ranges as well as Kurama. Hashirama can also perform Sage Art: Gate of the Great God, a technique that can bind a creature as powerful as the Ten-Tails.

Tailed Beast Influence
During his meeting with Nagato, Naruto was stabbed in the stomach with one of Nagato's chakra receivers. Nagato noted from that distance he would have been able to easily control the jinchūriki. However, while the young man was in Sage Mode, the fox's rage also spilled over into the form, and was reflected by Kurama's eyes being reflected in those of Naruto's Sage Mode: the irides turn red, and his pupil resembles that of both the fox's and his own when in Sage Mode, reminiscent of a cross. Some time later during Naruto's battle against the Ten-Tails' jinchūriki Obito Uchiha, Naruto once again used these two powers to create powerful attacks bolstered by the natural energy from Sage Mode even going as far as to mix natural energy with the Tailed Beast Ball. In this form, Naruto's eyes take on the same appearance as before. The markings of the Sage are also reflected on Kurama's Tailed Beast Mode shroud. After gaining a portion of chakra from all the nine tailed beasts and having Yin-Kurama sealed into him by Obito, Naruto becomes able to enter Sage Mode without manifesting the pigmentation around his eyes, though the pupils still take on a cross-like shape. In this state, Naruto is able to combine senjutsu with the tailed beasts' chakra to create powerful techniques like the Sage Art: Lava Release Rasenshuriken.

Trivia

 * In the film Naruto 5: Blood Prison, the red coat Naruto wore during his return in the Invasion of Pain would appear when he enters Sage Mode. Like every other physical change with the form, it would disappear when he turns back to normal.
 * The manifestation of animal like traits through the activation of natural energy and the performance of senjutsu, is a reference to the Jiraiya Ninja folk tale, where Jiraiya, Tsunade, and Orochimaru came to study specific animal magic into their ninjutsu use.