The most prominent part of the Teenage Mutant Ninja Turtles’ identities is their training in the martial arts since the day they mutated. However, the exact type wasn’t always told to the audience in each incarnation. Additionally, even if viewers were aware that the Turtles are students of ninjutsu, that word itself might sound like something fake created specifically for a television show involving giant green mutants.
Ninjutsu was a very real art, and its essence lives on through several modern descendants (such as bujinkan, nindo ryu bujutsu kai, and akban). The traditional form has been scattered to the wind, but its principles and history remain intact. Ninjutsu relies heavily on unconventional strategy and tactics, including prevalent use of espionage. Those who practiced the art were masters of stealth, working as scouts, assassins, and spies at least 1,500 years ago. Those skills made them key players in feudal Japan, and left a legacy widely mimicked and adapted in different aspects of all cultures.
Ninjutsu developed several different schools even during its own hey-day, and while the traditional art was modified by each to fit specific needs, the descendants reveal that a master of ninjutsu excelled at 18 special skills: seishinteki kyoukou (spirit refinement); taijutsu (hand-to-hand fighting); kenjutsu (sword); boujutsu (staff); soujutsu (spear); naginatajutsu (naginata); kusarigamajutsu (kusarigama); shurikenjutsu (throwing weapons); kayakujutsu (pyrotechnics); hensoujutsu (disguise); shinobi-iri (stealth); bajutsu (horsemanship); sui-ren (water skills); bouryaku (tactics); chouhou (espionage); intonjutsu (escape); tenmon (meteorology); and chimon (geography). Ninja trained their entire lives to be strong in all of these skills, learning to use any means available to survive even the most desperate situations.
Understanding the basic history of ninjutsu helps make sense of some widely accepted plot points the TMNT 
franchise has put forth since Day One. Given the skill level he shows time and again, Splinter is purportedly a master of ninjutsu. He has exceptional stealth capabilities, knows how to handle all the weapons (and then some) of each individual Turtle, and has shown to be adept at understanding any situation with barely a glimpse at the big picture (see TMNT II, and his sudden production of the TGRI bottle mere minutes after the company is introduced). If the audience accepts that he is a master of this complex art, it even explains one of his origin stories over the other. (Competing tales have him mutating from Hamato Yoshi’s pet rat or mutating from Yoshi himself.) No matter how much disbelief is suspended, it makes more sense that a man mastered ninjutsu before becoming a rat.
Knowing that there are 18 skills involved in mastering the art helps explain why each Turtle specializes in certain areas. Like any other multi-faceted artform or skill, different individuals will excel at different parts. Leonardo clearly mastered kenjutsu and shinobi-iri; but Donatello is arguably the best at tenmon and chimon (as the brains of the group), while also being the boujutsu master. The lack of some of these skills in all of them also helps to sell the idea that they are actually teenagers (something that was not accomplished by the movie franchise or the 2003 animated series). Adolescents aren’t going to necessarily understand the importance of hensoujutsu or chouhou when they just want to enjoy all that the world has to offer.
  There is a lot more to being sufficiently trained in ninjutsu than any TMNT incarnation explained. However, its complexity does help to resolve some conflicting and fuzzy plot elements that were essential to the franchise’s concept. Knowing that there are so many facets to being a true ninja, it’s easy to see which areas each Turtle had a knack for, and each particular expertise can be explored further, adding new depth to the character that helped make it famous.
