The massively popular martial art of Judo is derived from Jujitsu (also referred to as Jujutsu or Jiu-jitsu), an ancient system of hand-to-hand combat practiced by highly skilled samurai and ninja warriors on the battlefields of feudal Japan.

Beginning in the early 1880s, the Jujitsu fighter and instructor Jiguro Kano developed a new martial art based on Jujitsu techniques, with one organizing principle: to make the most efficient use of mental and physical energy. Kano called the system Judo, or gentle way, and saw it as not merely a self-defence method, but also a lifelong art. In 1882, Kano founded the Kodokan Judo Institute in Tokyo, which remains the top Judo authority in the world.

Kano later travelled to Europe and used his fluency in English to spread the practice and philosophy of Judo to a new Western audience. He also broke with Japanese martial arts tradition by taking on a female student, Sueko Ashiya, and opening a dojo, or training centre, for women in Tokyo. After Kano died in 1938, Judo only continued to grow in popularity. The first international tournament took place in 1947 between British and French fighters. In 1964, Judo became an Olympic sport, and it is now practiced by more than 8 million people around the world.

Because of Kano's belief that Judo was a way of life, training for this martial art is based on mental and moral development, in addition to physical. Two key goals for Judo fighters are hontai (a permanent state of alertness) and bonno (a disciplined mind, serene calm, control of the body, and readiness to react to any situation). The fundamental physical technique at the heart of Judo is using the opponent's body weight against her or him; in this way, a small fighter can beat a much larger, stronger opponent with the proper strategy, skill and technique.

Judo is practiced on mats, and there is no kicking or punching. The key moves in Judo are throws and grappling, including chokeholds, joint locks and armbars. Judo fighters are ranked according to a belt system, where each rank is called a kyu and the belts are called obi. Earning a black belt is considered a sign of proficiency and the beginning of a more advanced study; there are ten degrees, or dan, of black belt, in the Judo system. The Judo belt system has been adapted by many of the other modern martial arts, including Karate and Aikido.