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.

Usage
A prerequisite to being able to use senjutsu is that the user must have a very large reservoir of chakra themselves in order to manipulate the natural energy.

Advantages
Taught and learned by the toads, the advantages afforded to those who study senjutsu from the toads include:
 * The user's physical strength, speed, stamina, reflexes, perception and durability dramatically increase.
 * The user's ninjutsu, genjutsu, and taijutsu become more powerful.
 * The user can harness the natural energy surrounding them, 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, these include:
 * If the user draws in too much natural energy, they run the risk of transforming into a toad, 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 Toad 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 (and the imperfect nature of his transformation), 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, 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". 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.

Perfect Toad Sage Mode
Unlike Jiraiya, Naruto Uzumaki is able to perfectly balance the natural energy with his own chakra. This allows him to use Sage Mode in perfect balance, which allows him to retain all of his normal physical characteristics except for toad-like eyes: yellow irides, horizontal bar-like pupils, and reddish-orange pigmentation around the eyes; the mark of a true sage on each eye. A true sage would still suffer from the weaknesses of Sage Mode, requiring that they use Sage Art: Amphibian Technique in a battle situation. Because of the animosity of the Nine-Tailed Demon Fox sealed within Naruto, however, Fukasaku was rejected whenever he tried to fuse with him.

As a 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's skill in senjutsu has increased to the point that it doesn't take him nearly as long to enter Sage Mode.

During his meeting with Nagato, Naruto was stabbed in the stomach with Nagato's chakra disruption blade. Nagato thought that he would be able to control Naruto with his own chakra, but Naruto revealed a fusion of his initial jinchūriki form and Sage Mode characteristics. It was strong enough to completely resist Nagato's control at a close proximity. The irides turn red and his pupil resembles that of both the fox's and his own when in Sage Mode, like a cross. Naruto later shows off this form again against Obito Uchiha who had became the Ten-Tails' jinchūriki, and could power up his attacks to overpower Obito's durable chakra arms and even mix natural energy with the Tailed Beast Ball.

Advantages
Senjutsu which can also be studied from the snakes of the Ryūchi Cave also gain a number of advantages while using Sage Mode in battle, these include:
 * The user's perception abilities, reflexes, strength, speed, and stamina dramatically increase.
 * The user's ninjutsu, genjutsu and taijutsu becomes more powerful.
 * The user gains further access to snake anatomy, such as their brille, which can be used to lock out light as well as stop visual-based genjutsu from affecting them. They are also able to perform abilities that are characteristic of snakes, such as dislocating their jaw, although this is not exclusive to this mode.

Disadvantages
While mobile, the user is rendered incapable of gathering natural energy while remaining still in battle would leave them open to attack. 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 one weakness and metaphorically proclaimed that he transcended from snake to dragon.

Forms
While retaining most of his normal physical characteristics in Sage Mode, Kabuto Yakushi did display greater areas of darkened pigmentation of the markings around his eyes which tapered off after reaching his shoulder blades. More noticeably, he gained four horns growing out from the back of his head as well as darkened sclerae.

Advantages
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.

Disadvantages
The user has the disadvantage of having to stay still for a certain amount of time so they can gather enough natural energy to form the special senjutsu chakra. Thus, the user is left vulnerable while doing this. Unlike Toad Sage Mode, this version of Sage Mode doesn't carry over to any clones, which are created in base form.

Forms
In this mode, he retains all of his normal physical characteristics. He gains, however, a red pigmentation around his eyes, and his face gains several markings inclusive of a circular one in the middle of his forehead.

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.