The vast majority of martial arts will not work in self defense.
Many of them were not made for self defense in the first place, most provide no education in or understanding of real violence or how to prevent it, and a very large number of them utilize training methods that do not and cannot lead to real skills. Due to unrealistic training methods, these martial arts use techniques that will not work against a fully resisting, uncooperative opponent. In addition, most styles lack realistic training with and against modern weapons. In this article I'll explain why most martial arts don't work in self defense, and what is required for them to do so.
A great many martial arts were never made for self defense; some purposefully, some not. Tai chi for example, was not created for self defense. Nothing in the practice remotely resembles an assault or fight. Kenjutsu trains the use of samurai swords. Tae kwan do is a sport, with very limiting rules. Although some train aikido as a self defense system, does this look like realistic self defense?
Granted, the above is an extreme example, but nevertheless real attackers do not attack like you see above, nor flip through the air at the slightest touch...or even without being touched for that matter.
Some styles that were not created for self defense, purposefully or otherwise, do have techniques that can be made to work. However, everything about these styles aside from those isolated techniques goes against the principles required to attain real self defense skills.
It's rare to find a martial art that even mentions why, where, how, and who violence happens to. (Find out here.) It doesn't take long, but this needs to be the first step in any self defense system. Whether it's individual self defense, a domestic violence situation, or a war, a practitioner needs to understand what they're up against to effectively learn to defend against it. Prevention strategies can and should account for most self defense. They're nonexistent in most martial art training.
Very few traditional martial arts provide comprehensive, quality training methods. In many karate, tae kwan do, and kung fu schools, the primary training method is solo, prearranged forms performed in the air. This method of training alone will prepare a student for a real assault only a little better than ballet classes. Students must spend time striking and wrestling with other humans, getting hit and wrestled, and avoiding these things. And this training must be against completely uncooperative, fully resisting opponents. Cooperative training, no matter how hard, will not prepare a student for the chaos of a real attack.
Many training methods that look good on the surface are useless at best and counter productive at worse. Take a look at this very common "stick fighting" drill done in Filipino martial arts:
No one fights with sticks like this. But it's just a training drill, right? The practitioners are getting accustomed to attacking and blocking, right? Wrong. Look a little closer, and notice that no one is following through on their strikes like a real attacker would. Thus, the blocks these students are learning to use would be ineffective against a real attacker swinging a stick hard and fast. The positions they're learning to put their hands in will get their hands nailed by their opponent's sticks in reality. Not only is this drill useless, but it's teaching the practitioners to do things that will get them injured in a real stick fight. The same can and will be seen in martial arts schools around the world.
Isolated drilling of a technique is not only fine, but necessary. The problem isn't the isolation, it's when the attack doesn't resemble a real attack, and the defense doesn't resemble a real, effective defense. When a technique is isolated in training, practitioners must be able to do the technique with full speed and power, and the defense must still work. Otherwise, the training is creating bad habits. Unfortunately, this is standard operating procedure at most martial arts schools.
Largely due to poor training, most martial arts are filled with sub-optimal to horribly dangerous techniques. (See this page for requirements for effective self defense techniques.) These include karate and kung fu style punches and blocks that leave practitioners wide open to counter strikes...
...to crazy aikido throws and wing chun complex trapping. Because most traditional martial art training is not realistic, the practitioners never realize the techniques they're learning aren't realistic either; until they actually need them.
Even with good training and techniques, strategy is essential. Most martial art schools do not promote or consider various strategies for self defense. When is it time to attack? When is it time to run? What type of defense is most appropriate for a small woman vs. a large man? Because the majority of martial art training follows a strict, stylized curriculum, there is no room for strategic or technical differences in size, strength, gender, and disposition.
These days, an attacker is likely to have a weapon. The vast majority of martial art schools do not train realistic defense against modern weapons, if they train with them at all. And even fewer train practitioners to use modern weapons. Those that do, especially in the Filipino martial arts, tend to be the worst regarding realistic training and ineffective techniques. Quality weapons training must be a part of every self defense system.
In addition to poor techniques, training methods, and strategies, many martial arts go far beyond in regards to guru cults and pure madness. See our article on The Intersection Of Martial Arts And Religion for more.
Note: To learn the most realistic, functional martial arts techniques see The Ultimate Guide to Unarmed Self Defense.