My teacher (Takamatsu toshitsugu) once described ninjutsu to me (Masaaki Hatsumi) as follows: “It is said that ninja knew all the martial ways. In each, they would undergo at least the minimum training essential to their life as a ninja. They would study the eight branches of the Ninja Hachimon (忍者八問): Ninja kiai (energy harmonization), Koppotaijutsu, Ninpo swordwork, spearwork, Shuriken, fire, traditional arts, and general knowledge.”
In other words, life of a Shinobi started with the Ninja hachimon. Over time, however, this changed so that greater weight was placed on Happo Hiken. the Shinobi Happo Hiken consists of the following:
- Taijutsu, Hichojutsu, nawa-nage (body skills and rope throwing)
- Karate Koppojutsu Taijutsu, Jutaijutsu (unarmed fighting)
- Sojutsu, naginatajutsu (spar and halberd arts)
- Bojutsu, Jojutsu, Hanbojutsu (staff and stick arts)
- Senban nage, Ken Nagejutsu, Shuriken (throwing blades)
- kajutsu, Suijutsu (use of fire and water)
- Chikujou Gunryaku Hyoho (military fortification, strategy and tactics.)
- Onshinjutsu (concealment)
The branches listed above were known as Happo (eight methods), and were supplemented by Hikenjutsu (secret sword arts; 秘剣), in other words, the shinobi sword, kodachi, jutte, and tessen, to complete the Ninja Happo Hiken.
In later periods, the term Togakure-ryu Juhakkei was also used. the eighteen forms of the Shinobi were defined as [above… ]. These were obliterated (破) by, or rather concealed within, the Bugei Juhappan (武芸十八般), thereby escaping through transformation into thirty-six forms: discretion became the better part of valor. In a sense, the evolved into the thirty-six Togakure strategies , the Kuji (九字) of the Santo Tonko (鼠逃遁甲) techniques, and even the Juji (十字) principle of bonding with the divine.
In fact, Ninja did not simply learn all forms of martial arts through their training: they continued studying until they reached a level far beyond mere technical prowess. 
 Masaaki Hatsumi. Way of the Ninja. (2004). p. 21-22.