In Japanese history, a ninja (忍者 ) was a warrior specially trained in a variety of unorthodox arts of war. These include assassination, illusion, and espionage.

The exact origin of the ninja is a matter of debate. It is known that ninja appeared in 14th century Japan and remained active from the Kamakura to the Edo period. The role of the ninja may have included sabotage, espionage, and scouting. Such actions may have taken place at the service of a feudal lord (daimyo, shogun), or other entity waging guerilla warfare.

Kōga-ryū (甲賀流, literally “the Kōka School” and originally pronounced Kōka-ryū) is a school of ninjutsu. It is one of the most well-known schools of ninjutsu, along with Iga-ryū. It originated from the town of Kōka, modern Kōka City in Shiga Prefecture. This school of ninjutsu specializes in the use of guerrilla warfare tactics and the use of chemicals, a vestige of which can be seen in the many pharmaceutical companies in modern Kōka City.

Koga Ryu Ninjutsu was founded by 53 families during the 10th Century, probably under the Tenkyo period (938-946). Mochizuki Saburo Kameie was given an area of land called Kogagun in virtue of his performance in the battle against Taira No Masakado. He changed his name to Koga Oni No Kami Kameie. However, it was his son Oni No Kami Iechika who studied Genjutsu under Tatsumaki Hoshi and was the founder of Koga Ryu. The families with the most important in Koga Ryu were Koga, Mochizuki, Ugai, Naikii, Akutagawa, Ueno, Ban and Nagano.

Here are a list of links that contain much information about the Mochizuki name in Shinobi history.

• “Ninjutsu hisho ogiden no kan” ( the secret essence of ninja technique) was originally composed by Mochizuki Shigeie in 1536. Mochizuki Shigeie was a Koga Ryu Ninja of the Mochizuki family and was captain of Koga Go Jusan Ke (the 53 families of Koga Ryu).
• Koga-ryu Ninjutsu Yashiki is the only existing residence of Ninja. In the Edo Period this residence was built by Mochizuki Izumonokami, the head of the 53 Kaga warrior houses of Koga-ryu, one of the major styles of ninjutsu.
• The Ninjutsu Thesis of Iga Ryu & Koga Ryu.
• History of the Koga Ryu.
• Koka Ninja House Photo Tour. Koka Ninja House (Koka-ryu Ninjutsu Yashiki) is the former residence of Mochizuki Izumonokami, the leading Koga ninja family of the 53 Koka ninja families. The house is in its original location in the Koka area of Shiga Prefecture.