The rules for modern mixed martial arts competitions have changed significantly since the early days of vale tudo, Japanese shoot wrestling, and UFC 1, and even more from the historic style of pankration. As the knowledge of fighting techniques spread among fighters and spectators, it became clear that the original minimalist rule systems needed to be amended. The main motivations for these rule changes were protection of the health of the fighters, the desire to shed the perception of "barbarism and lawlessness", and to be recognized as a legitimate sport.

The new rules included the introduction of weight classes; as knowledge about submissions spread, differences in weight had become a significant factor. There are nine different weight classes in the Unified Rules of Mixed Martial Arts. These nine weight classes include flyweight (up to 125 lb / 56.7 kg), bantamweight (up to 135 lb / 61.2 kg), featherweight (up to 145 lb / 65.8 kg), lightweight (up to 155 lb / 70.3 kg), welterweight (up to 170 lb / 77.1 kg), middleweight (up to 185 lb / 83.9 kg), light heavyweight (up to 205 lb / 93.0 kg), heavyweight (up to 265 lb / 120.2 kg), and super heavyweight with no upper weight limit.

Small, open-fingered gloves were introduced to protect fists, reduce the occurrence of cuts (and stoppages due to cuts) and encourage fighters to use their hands for striking to allow more captivating matches. Gloves were first made mandatory in Japan's Shooto promotion and were later adopted by the UFC as it developed into a regulated sport. Most professional fights have the fighters wear 4 oz gloves, whereas some jurisdictions require amateurs to wear a slightly heavier 6 oz glove for more protection for the hands and wrists.

Time limits were established to avoid long fights with little action where competitors conserved their strength. Matches without time limits also complicated the airing of live events. The time limits in most professional fights are three 5 minute rounds, and championship fights are normally five 5 minute rounds. Similar motivations produced the "stand up" rule, where the referee can stand fighters up if it is perceived that both are resting on the ground or not advancing toward a dominant position.

In the U.S., state athletic and boxing commissions have played a crucial role in the introduction of additional rules because they oversee MMA in a similar fashion to boxing. In Japan and most of Europe, there is no regulating authority over competitions, so these organizations have greater freedom in rule development and event structure.
Victory

Victory in a match is normally gained either by the judges' decision after an allotted amount of time has elapsed, a stoppage by the referee (for example if a competitor can not defend himself intelligently) or the fight doctor (due to an injury), a submission, by a competitor's cornerman throwing in the towel, or by knockout.

Knockout (KO): as soon as a fighter becomes unconscious due to strikes, his opponent is declared the winner. As MMA rules allow ground fighting, the fight is stopped to prevent further injury to an unconscious fighter.

Submission: a fighter may admit defeat during a match by:

    a tap on the opponent's body or mat/floor
    a verbal announcement/verbal tap

Technical Knockout (TKO)

    Referee stoppage: The ref may stop a match in progress if:
        a fighter becomes dominant to the point where the opponent can not intelligently defend himself and is taking excessive damage as a result
        a fighter appears to be unconscious from a submission hold or due to a strike
        a fighter appears to have a significant injury such as a cut or a broken bone

Doctor Stoppage: the referee will call for a time out if a fighter's ability to continue is in question as a result of apparent injuries, such as a large cut. The ring doctor will inspect the fighter and stop the match if the fighter is deemed unable to continue safely, rendering the opponent the winner. However, if the match is stopped as a result of an injury from illegal actions by the opponent, either a disqualification or no contest will be issued instead.

Corner stoppage: a fighter's corner men may announce defeat on the fighter's behalf by throwing in the towel during the match in progress or between rounds.

Decision: if the match goes the distance, then the outcome of the bout is determined by three judges. The judging criteria are organization-specific.

Forfeit: a fighter or his representative may forfeit a match prior to the beginning of the match, thereby losing the match.

Disqualification: a "warning" will be given when a fighter commits a foul or illegal action or does not follow the referee's instruction. Three warnings will result in a disqualification. Moreover, if a fighter is injured and unable to continue due to a deliberate illegal technique from his opponent, the opponent will be disqualified.

No Contest: in the event that both fighters commit a violation of the rules, or a fighter is unable to continue due to an injury from an accidental illegal technique, the match will be declared a "No Contest".