The traditional rules of judo are intended to avoid injuries to the participants and ensure proper etiquette. Some later additions to the rules were motivated by a desire to make the sport more interesting to observers.

Penalties may be given for being inactive during the match, or for using illegal techniques. Fighting must be stopped if a participant is outside the designated area on the mat (tatami). If the referee and judges need to discuss something during groundwork, the referee will call sonomama (which means "do not move") and both fighters must stop in the position they are in. When they are done, the referee says yoshi and the match continues.
All scores and penalties are given by the referee. The judges can make a decision that changes the score or penalty given by the referee.
There are slight differences to IJF rules to accommodate blind judo.

Competition scoring

Judo has three grades of score: ippon, waza-ari and yuko. An ippon literally means "one point" and wins the match. Scoring is lexicographic; a waza-ari beats any number of yuko, but a waza-ari and a yuko beat a waza-ari with no yuko.
When the koka was in use it was a lower score that yuko. A yuko beats any number of koka.
A fifteen-second hold down scores yuko. If the person who secured the hold down already has a waza-ari, they only need to hold the hold down for twenty seconds to score ippon by way of two waza-ari (waza-ari-awasete-ippon). Throws further lacking the requirements of an ippon or a waza-ari might score a yuko. If the scores are identical at the end of the match, the contest is resolved by the Golden Score rule. Golden Score is a sudden death situation where the clock is reset to match-time, and the first contestant to achieve any score wins.

Representation of scores

Judo scoreboards show the number of waza-ari and yuko scores scored by each player. Some computerised scoreboards will briefly indicate that an ippon has been scored.
Scoreboards normally also show the number of penalties imposed on each player, and sometimes the number of medical visits for each. Electronic scoreboards also usually include timers for measuring both competition time and osekomi time.
"Koka" was the score of 1/8 point, until the new regulation changed.

Penalties

First penalty given will be a warning which is noted on scoreboard. Second penalty will be scored as "yuko" for the opponent. Third penalty will be scored as "waza-ari". The fourth penalty is called "hansoku make". It will be scored as an "ippon" for the opponent. With an "hansoku make" the match will end permanently. The player who got "hansoku make" will be disqualified. One can also get a direct "hansoku make" for more serious incidents.

Judo has formed the basis for many military combatives and defensive tactics training around the world. The Japanese police have trained in Judo since 1886, when Judo (at the time known merely as Kano Jujitsu) defeated several other established schools of Jujitsu in a tournament.

In addition to the above, Judo's background in traditional Jujitsu combined with its police and military applications, has resulted in kata specifically designed to teach technical principles for self-defense: Kime No Kata (Forms of Decision) and Kodokan Goshin Jutsu (Forms of Self Defense).
Various aspects of Judo principles and training methods promote attributes and skills helpful in self-defense:

Training with full power and speed against fully-resisting opponents: builds speed, stamina, strength, and tenacity.

Body and mental conditioning by repeatedly being thrown with significant force.
Training in safe methods to take falls.

Ability to accurately and quickly use balance, distance, and timing against skilled opponents in fully-resistive sparring. Judo practitioners are experts in controlling their opponent's balance whilst maintaining their own.
Sports Judo rules emphasize rapid transition to pins or submissions after a take-down, which builds skills in explosive use of chokes and locks in self-defense situations.

Emphasis in controlling one's opponent during throws allow a practitioner to dictate the angle, direction, and force with which his or her opponent lands on the ground. The consequences could be gentle or lethal, depending on the Judo practitioner's intentions.

However, there are some criticisms about the use of Judo for self-defense training:
Over-reliance on using the Judo-gi (clothing): In order to train Judo for self-defense, it is necessary to have some experience in sparring against partners who are not wearing a gi. Fortunately, the amount of adaptation needed is minor,[citation needed] and many techniques do not strictly rely on grabbing the gi; indeed, some techniques (especially in groundwork or Ne Waza) do not use the gi at all.

Over-emphasis on the rules of sports Judo: Some Judo clubs or instructors teach Judo strictly in the context of sport.

Lack of striking techniques: Striking techniques in Judo are usually only taught to dan-grades (ie. black belts) for demonstrations and kata.

There are two divisions of rank: below-black-belt-level "grades" (kyū), and black-belt-level "degrees" (dan). There are ordinarily 10 dan ranks, which are ranked in ascending numerical order, though in principle there is no limit to the number of dan ranks.

Belt colours

For dan ranks, the first five are coloured black, 6th, 7th, and 8th dan have alternating red and white panels, and for 9th and 10th dan the belts are solid red.[30] However, holders of grades above godan (5th dan) will often wear a plain black belt in regular training.

Some countries also use colored tips on belts, to indicate junior age groups. Historically, women's belts had a white stripe along the centre.[citation needed]
The kyū ranks are normally awarded by local instructors (sensei), but dan ranks are usually awarded only after an exam supervised by independent judges from a national judo association.

Brazil

Brazilian belt rankings are normally white, blue, yellow, orange, green, purple, brown and black. Additionally, a grey belt may be given to very young judoka (under 11 or 13 years old) just before the blue. Competitors are organised into two categories depending on grading; the first is from white to green, and the second is purple through black.

Canada

In Canada belt rankings for Seniors are, in ascending order: white, yellow, orange, green, blue, brown and finally black. Belt rankings for Juniors use white-red, white, white-yellow, yellow, yellow-orange, orange, red- green, green-blue, blue, blue-brown, and brown.

United States

In the US only senior players (adults, usually those age 16 and over) are allowed to earn dan levels, signified by wearing a black belt. The USJF and USJA recognise dan grades awarded by the other organization. Advanced kyū levels can be earned by both seniors and juniors (children under the age of about 16) and are signified by wearing belts of various colours other than black. The order of belt colours can vary from dojo to dojo, depending on the dojo's organizational affiliation.