Judo (Japanese: 柔道, jūdō; "gentle way") is a martial art, sport, and philosophy originated in Japan. Judo was developed from Jujutsu, and was founded by Dr. Jigoro Kano (嘉納治五郎) in 1882. The sport became the model of the modern Japanese martial arts, gendai budo, developed from old koryu schools. Practitioners of Judo are called judoka.

While Judo includes a variety of rolls, falls, throws, pins, chokes, joint-locks, and methods of percussion, the primary focus is on throwing (nage-waza, 投げ技), and groundwork (ne-waza,寝技). Nage-waza is divided in two groups of techniques, standing techniques (tachi-waza, 立技) and sacrifice techniques (sutemi-waza, 捨身技). Standing techniques are divided in hand techniques (te-waza, 手技), hip techniques (koshi-waza, 腰技) and foot/leg techniques (ashi-waza, 足技). Sacrifice techniques are divided into those in which the thrower falls directly backwards (ma-sutemi-waza, 真捨身技) and those in which he falls onto his side (yoko-sutemi-waza, 橫捨身技).

The groundwork techniques are divided into: attacks against the joints or joint locks (kansetsu-waza, 関節技), strangleholds or chokeholds (shime-waza, 絞技), and holding or pinning techniques (osaekomi-waza, 押込技).

A kind of sparring is practiced in judo, known as randori (乱取り), meaning "free practice". In randori, players (known as judoka) may attack each other with any judo throw or grappling technique. Striking techniques (called atemi-waza) such as kicking and punching, along with knife and sword techniques are retained in the katas taught to higher ranking judoka (for instance, in the kime-no-kata), but are forbidden in contest (and usually prohibited in randori), for reasons of safety. Also for reasons of safety, chokeholds, jointlocking - and the sacrifice (sutemi) techniques, which can be very spectacular, are often subject to age and/or rank restrictions; in the United States, one must be 13 or older to use chokeholds, and 17 or older, or hold the rank of Shodan (first grade black belt) or higher to use armlocks.

In randori and shiai (tournament) practice, when an opponent successfully executes a chokehold or joint lock, one "taps out" by tapping the mat or one's opponent at least twice in a manner that clearly indicates the submission. When this occurs, the match is over, and the tapping player has lost, but the chokehold or joint lock ceases. Because this allows a merciful exit to the match, injuries related to these holds are quite rare.