Ninjutsu (ninja art) (忍術)

Ninjutsu is a generic term for techniques used by Ninja (persons in feudal Japan who used ninjutsu for the purposes of espionage, assassination, sabotage, etc.), who were active during the Muromachi Period and the Sengoku Period, to gather intelligence or to destabilize intelligence against the enemy camp and also arts of warfare and martial arts necessary to carry out Ninja's operations. Ninjutsu includes various martial arts such as swordplay and Jujutsu but equipment (weapons) specific to Ninjutsu is referred to as Ningu or Ninki. Additionally, teachings of some schools of Japanese martial arts included Ninjutsu.

Summary
Techniques to slip into the enemy camp to gather intelligence are described in Ninjutsu-sho (book of Ninjutsu) such as Mansei (or Bansei) Shukai and Shonin-ki. Koga school that fought against the Muromachi bakufu (Japanese feudal government headed by a shogun) and Iga school that was famous for Hanzo HATTORI, who was a retainer of Ieyasu TOKUGAWA are well known.

The main existing schools of Ninjutsu
It is said that as many as 49 schools of Ninjutsu used to exist in Japan.
The schools of Ninja included the following:

'Nakagawa school' in Aomori Prefecture
'Haguro school' in Yamagata Prefecture
'Uesugu school' and 'Kaji school' in Niigata Prefecture
Fukuchi school' and 'Matsumoto school' in Tochigi Prefecture
'Akutagawa school,' 'Aoki school,' 'Togakure school' and 'Ito school' in Nagano Prefecture
'Koyo school,' 'Takeda school,' 'Matsuda school,' 'Ninko ("忍甲") school' and 'Ninko ("忍光") school' in Yamanashi Prefecture
'Hojo school' in Kanagawa Prefecture
Akiba school' and 'Ichizen school' in Aichi Prefecture
'Muhyoshi school' in Ishikawa Prefecture
'Yoshitsune school' in Fukui Prefecture
'Rikyoku school,' 'Tsuji-Ichimu school,' 'Hattori school,' 'Taki school,' 'Yoshimori school,' 'Uchigawa school,' 'Takino school' and 'Sawa school' and 'Iga school' in Mie Prefecture
'Koga school' in Shiga Prefecture
'Asuka school,' 'Hidesato school,' 'Kyushu school,' 'Gamo school' and 'Kusunoki school' in Nara Prefecture
'Natori school,' 'Shinkusunoki school,' 'Saika school,' 'Negoro school' and 'Kishu school' in Wakayama Prefecture
'Bizen school' and 'Kamiizumi school' in Okayama Prefecture
'Fukushima school' and 'Hikko school' in Hiroshima Prefecture
'Mikumo school' and 'Iga school' in Kochi Prefecture
'Kuroda school' in Fukuoka Prefecture
'Nanban school' in Nagasaki Prefecture
'Oe school' and 'Yahata school' in Kumamoto Prefecture
'Kurama Yoshin school' and 'Shiota Yoshin school' in Kagoshima Prefecture

In addition, 'Gyokushin-ryu school' and 'Kumogakure-ryu school' also existed.

Organizations that teach Ninjutsu
Bujinkan Training Hall
Banke Shinobi no Den Kenshujo
Jissen Kobudo (actual fighting of old budo school) Shizenkan
Genbukan Sekai Nimpo Bugei remmei (Genbukan Worldwide Nimpo Bugei League)
Trivia
According to the scientific analysis performed by a TV show abroad that was introduced in a Japanese TV show "Sekai Marumie Terebi-tokusobu," Ninjutsu was proven to be the world's fiercest martial art.