Brazilian Jiu-Jitsu (BJJ) is a martial art and combat sport that focuses primarily on ground fighting. BJJ is a derivative of early 20th century Kodokan Judo. Like judo, BJJ promotes the principle that a smaller, weaker person can successfully defend themselves against a bigger, stronger assailant using leverage and proper technique; applying joint-locks and chokeholds to defeat them. The premise is that most of the advantage of a larger, stronger opponent comes from superior reach and more powerful strikes, both of which are somewhat negated when grappling on the ground.

BJJ permits a wide variety of techniques to take the fight to the ground after the initial grip or clinch. Once the opponent is on the ground, a number of maneuvers (and counter-maneuvers) are available to manipulate the opponent into a suitable position for the application of a submission technique.

Achieving a dominant position on the ground is one of the hallmarks of the BJJ style, and includes effective use of the guard position to defend oneself from bottom, and passing the guard to dominate from top position with side control, mount, and back mount positions.

Joint locks typically involve isolating an opponent's limb and creating a lever with the body position which will force the joint to move past its normal range of motion. Pressure is increased in a controlled manner and released if the opponent cannot escape the hold and signals defeat by submitting. Opponents can indicate submission verbally or they can tap out (i.e. tap the opponent or the mat several times). A choke hold, disrupting the blood supply to the brain, can cause unconsciousness if the opponent does not submit soon enough. A less common type of submission hold is a compression lock, where the muscle of an opponent is compressed against a hard, large bone (commonly the shin or wrist), thus causing significant pain to the opponent. This type of lock often also hyper-extends the joint in the opposite direction, pulling it apart.

Training methods include drills in which techniques are practiced against a non-resisting partner; isolation sparring, commonly referred to as positional drilling, where only a certain technique or sets of techniques are used; and full sparring in which each opponent tries to submit their opponent using any legal technique. Physical conditioning and coordination also play an important part of the training.

The standards for grading and belt promotions which measure a person's skill and rank in Brazilian Jiu-Jitsu are based upon the amount of technical knowledge they can demonstrate as well as their performance in sparring and competition. This allows for smaller and older practitioners to be recognized for their knowledge though they may not be the strongest fighters in the academy. It is a distinctly individual sport, and practitioners are encouraged to adapt the techniques to make them work for their body type, strategic preferences, and level of athleticism.

Competitions play an important role in Brazilian Jiu-Jitsu grading, as they allow an instructor to compare the level of his students against those of the same rank from other schools. A belt promotion may be given after success in a competition, particularly at the lower belt levels. A promotion might also be awarded when a person can submit most people in his school of the same rank, e.g. a white belt who consistently submits most other white belts in sparring and is starting to catch blue belts.

BJJ is not only a sport, it is also an excellent way of life that promotes physical fitness and builds character. Brazilian Jiu-Jitsu can be trained for self defense, sport grappling tournaments (gi and no-gi), and mixed martial arts (MMA) competitions.