Historical records state that certain individuals/families from the Iga/Koga (modern Mie/Omi) region were noted for possessing specific skills and were employed (by samurai) to apply those and other skills. These records, which were kept by people both within the region and outside of the region, refer to the individuals/families as "Iga/Koga no Mono" (Men of Iga/Koga) and "Iga/Koga no Bushi" (Warriors of Iga/Koga). Due to this region's terrain, it was largely unexplored, and the people living within lived a relatively isolated existence. This enabled them to develop perspectives, which differed from the "mainstream" society of the time, which was under the direct influence of the upper ruling classes. When necessary, they successfully used the superstitions of the masses as a tool/weapon and became feared and slightly mythologized because of this.

In the mid/late 1500's their difference in perspective led to conflict with the upper ruling classes and the eventual invasion/destruction of the villages and communities within the Iga/Koga region. The term "ninja" was not in use at this time, but was later introduced in the dramatic literature of the Tokugawa period (1605-1867). During this period, ancestral fears became contempt and the stereotypical image ("clans of assassins and mercenaries who used stealth, assassination, disguises, and other tricks to do their work") was formed which, to this day, is still very much the majority opinion.

Over 70 different "ninjutsu ryu" have been catalogued/identified, however, the majority of them have died out. Most were developed around a series of specific skills and techniques, and when the skills of a particular ryu were no longer in demand, the ryu would (usually) fade from existence. The three remaining ninjutsu ryu (Togakure ryu, Gyokushin ryu, and Kumogakure ryu) are encompassed in Dr. Masaaki Hatsumi's Bujinkan Budo Taijutsu system. These ryu, along with six other "bujutsu ryu" (Gyokko Ryu, Koto Ryu, Takagi Yoshin Ryu, Shinden Fudo Ryu, Gikan Ryu and Kukishinden Ryu), are taught as a collective body of knowledge (see Sub-Styles for other info). 

During the "Ninja-boom" of the 80's, instructors of "Ninjutsu" were popping out of the woodwork - it was fashionable to wear black. Now that the boom is over there are not as many people trying cash in on the popularity of this art. However, as with all martial arts, it would be wise to be very careful about people claiming to be "masters personally taught by the Grandmaster in Japan".

How do you verify the authenticity of an instructor? In the case of a Bujinkan Budo Taijutsu instructor there a few points which one can use. 

First: all recognized "instructors" of the Bujinkan Dojo will, in addition to their Dan grade (black belt), have either a Shidoshi-ho (assistant teacher - first to fourth Dan) or Shidoshi (teacher - fifth to ninth Dan) certificate/ license from Dr Hatsumi. Only people with these certificates are considered to be qualified to teach his system (a Dan grade alone DOES NOT make one a teacher).

Second: in addition to these certificates/licenses, all recognized "instructors" of the Bujinkan Dojo will possess a valid Bujinkan Hombu Dojo Shidoshi-kai (Bujinkan Headquarters Dojo Teachers Association) for the current year. These cards are issued each year from Dr Hatsumi to those recognized as "instructors".

These points will help you if you are looking at training with someone from the Bujinkan Dojo. Beyond that, it's a case of "buyer beware". 

To provide some kind of brief description, ninjutsu includes the study of both unarmed and armed combative techniques, strategy, philosophy, and history. In many Dojos the area of study is quite comprehensive. The idea being to become adept at many things, rather than specializing in only one. 

The main principles in combat are posture, distance, rhythm and flow. The practitioner responds to attacks in such a way that they place themselves in an advantageous position from which an effective response can be employed. They are taught to use their entire body for every movement/technique, to provide the most power and leverage. They will use the openings created by the opponent's movement to implement techniques, often causing the opponent to "run in/on to" body weapons.

Training progresses through skills in Taihenjutsu (Body changing skills), which include falling, rolling, leaping, posture, and avoidance; Dakentaijutsu (Striking weapons body techniques) using the entire body as a striking tool/weapon - how to apply and how to receive; and Jutaijutsu (Supple body techniques) locks, throws, chokes, holds - how to apply and how to escape. 

In the early stages, weapons training is usually limited to practicing how to avoid attacks - overcoming any fear of the object and understanding the dynamics of its use from the perspective of "defending against" (while unarmed). In the mid and later stages, once a grounding in Taijutsu body dynamics is in place, practitioners begin studying from the perspective of 'defending with' the various tools/weapons.

In the early stages of training, kata are provided as examples of 'what can be done here' and 'how to move the body to achieve this result'. However, as the practitioner progresses, they are encouraged to explore the openings which naturally appear in people's movements and apply spontaneous techniques based upon the principles contained within the kata. This free flowing style is one of the most important aspects of ninjutsu training. Adaptability is one of the main lessons of all of these ryu.

Due to the combative nature of the techniques studied, there are no tournaments or competitions in Ninjutsu. As tournament fighting has set rules which compel the competitor to study the techniques allowed within that framework, this limits not only the kinds of techniques that they study, but also the way in which they will apply those techniques. The way that you train is the way that you fight. Ninjutsu requires that its practitioners be open to any situation, and to be able to adapt their technique to ensure survival.

Dr. Masaaki Hatsumi has been the recipient of numerous cultural awards in recognition of his extra-ordinary knowledge of Japanese martial culture. He is considered by many to be the only source for authentic "ninjutsu". However, as was noted above, the teachings of the three ninjutsu ryu which are part of his Bujinkan system, are not taught individually. Rather, they are taught as part of the collective body of knowledge which forms the foundation of his Bujinkan Budo Taijutsu system. 

Shoto Tanemura, formerly of the Bujinkan Dojo, formed his own organization (Genbukan Dojo) and claimed to be the Grandmaster of/teaching both Iga and Koga Ryu Ninjutsu. He has since formed a number of other organizations and is becoming more widely known for his "Samurai Jujutsu" tapes (Panther Productions).

The list of names of people claiming to teach "Koga Ryu Nijutsu" is quite long. The last person to be recognized as part of the Koga Ryu lineage in Japan was Seiko Fujita. His knowledge of "ninjutsu" died with him - he left no successor.