Senjutsu

are a specialised field of jutsu that involve the use of natural energy. Senjutsu practitioners, known as sages, learn to draw natural energy inside their bodies, blending it with their own chakra to create. This chakra adds a new dimension of power to the user's techniques, while also allowing for the use of techniques that would not otherwise be possible.

Overview
Senjutsu is known to be taught at two locations: Mount Myōboku and Ryūchi Cave. To use senjutsu, one must first learn to sense the natural energy in their surroundings and gather it inside their body. It must then be balanced with the user's own physical and spiritual energies, which make up their chakra, to create senjutsu chakra. If too little natural energy is added to the mix, senjutsu cannot be used, but gathering too much results in the user turning to stone. In the case of humans, absorbing too much natural energy first causes them to transform into an animal, be it a toad for practitioners at Mount Myōboku or a snake for practitioners at Ryūchi Cave. This process can be reversed if the natural energy is expelled from the person's body before they fully transform, but once the transformation is complete, it becomes permanent.

Those who are successful in this training learn to enter a state called Sage Mode, in which their physical parameters and sensing capabilities are dramatically enhanced for as long as the mode is active. Though trainees seem to enter Sage Mode automatically during the learning process, it is possible to create senjutsu chakra without entering Sage Mode in order to store it for later. Whereas the use of ninjutsu exhausts the user due to expending their chakra, senjutsu takes in natural energy from the outside, and thus causes the user to recover more quickly instead. However, senjutsu can only be used by those who already possess an enormous amount of chakra, as otherwise the natural energy would immediately overwhelm them. Techniques made using senjutsu chakra cannot be nullified by Truth-Seeking Balls, and are also capable of harming the Ten-Tails' jinchūriki, whose body is immune to ninjutsu.

Generally, senjutsu can only by used by those who undergo extensive training, though there are methods of obtaining senjutsu chakra through other means:
 * Members of Jūgo's clan have a unique genetic makeup that causes their bodies to absorb natural energy and undergo transformations. However, the sudden influx of power also causes them to go berserk at times.
 * Orochimaru developed an enzyme from Jūgo's bodily fluids that is capable of replicating his power in others. This enzyme serves as the basis for his cursed seals, which feed on their bearer's chakra and release Orochimaru's senjutsu chakra in return.

Sage Power
In the anime, the toads of Mount Myōboku are shown to have been using an ability called since long before Hagoromo spread chakra among Earth's inhabitants. By gathering natural energy into their bodies, they were able to drastically increase their strength, as well as shoot jets of water from their mouths in a manner similar to Water Release techniques. Despite these ancient practitioners having no chakra with which to balance the natural energy they absorbed, using sage power appears to have carried similar risks to using senjutsu, as indicated by the petrified remains of those who failed to control it being present at the mountain. The toad Gamamaru eventually taught Hagoromo to use sage power, making him the first person to use senjutsu in its modern form.

Trivia

 * The word uses the same hiragana (せんじゅつ) as the Japanese art of, which may allude to a sage's ability to sense chakra while in Sage Mode.
 * In the anime, the sound of natural energy being gathered to form senjutsu chakra is the same as the one used for the healing green chakra of the Mystical Palm Technique.
 * Whereas natural energy itself is generally only perceptible to sages, sensor types are able to detect senjutsu chakra normally, though it noticeably alters its owner's chakra signature.
 * In the ninth Naruto OVA, use of senjutsu during the Chūnin Exams is against the rules due to it giving Naruto Uzumaki an unfair advantage.