"Only the enlightened ruler, the worthy general, can use the highest intelligence for spying, thereby achieving great success. Spies are a key element in warfare. On them depends an army's every move" - Sun Tzu's Art of War
Shinobi no jutsu or ninjutsu (忍術) is a set of arts that includes psychology, espionage, sabotage, murder, military science, divination, astrology, astronomy, magic, various kinds of observation skills, and any other skill shinobi needed to survive and achieve their aims. These might also include the martial arts, as a shinobi would have to train himself in arts such as jujutsu grappling, kenjutsu swordsmanship, archery and horse riding.
According to the Bansenshukai, shinobi no jutsu was also called kansai because kan means ‘wicked’ as it is used in kankyoku (evil-minded) or mokan (falsehood). Even though shinobi no jutsu looks normal on its surface, it always has a twisted and deeply plotted tactic beneath the surface. Even though a shinobi is on the righteous path of loyalty, his methods have corruption within them and are criminal, thus the negative word kansai was also used.
Manuscripts on the arts of the shinobi have only the simplest mention of astrological skills and the arts of astronomy, and assume that the reader should have a good understanding of these things. In particular, they have no mention of swordsmanship, horse-riding skills or other combat skills, because it was taken for granted that people would naturally train themselves in those areas. Simply looking at the shinobi scrolls limits our understanding of the full range of skills in which shinobi would have been proficient at. Most writings on the shinobi arts have only simplified information, and where it needs detailed explanations, the authors would write down "kuden" (passed down in oral traditions).
While there are several styles of "modern ninjutsu," the historical lineage of these styles is disputed. Some schools and masters claim to be "the only true and legitimate heirs" of the art, but their claims have never been proven true.