A yellow card is used in many sports as a means of cautioning a player regarding their conduct, or indicating that a player is to receive a certain level of punishment.

In association football (soccer), a yellow card is shown by the referee to indicate that a player has been officially cautioned. The player's details are then (traditionally) recorded by the referee in a small notebook; hence a caution is also known as a booking.

A player who has been cautioned may continue playing in the game. However, a player who receives a second caution in a match is sent off (shown first the yellow card again, and then a red card), meaning that he must leave the field immediately, take no further part in the game, and that he may not be replaced.

Law XII of the Laws of the Game (which are set by the International Football Association Board and used by FIFA) lists the categories of offences that may result in a caution. Broadly these are:

1. Unsporting behaviour
2. Dissent by word or action
3. Persistently infringing the Laws of the Game
4. Delaying the restart of play
5. Failing to respect the required distance of a corner kick or free kick
6. Entering or re-entering the field of play without the referee’s permission
7. Deliberately leaving the field of play without the referee’s permission

Note that these are very broad categories, and the referee has a very large degree of discretion as to whether an act constitutes a cautionable offence. Other Laws may specify circumstances under which a caution should or must be given, and numerous directives to referees also provide guidance. A controversial change to the Laws of the Game championed by FIFA President Sepp Blatter mandated automatic yellow cards for players who remove their shirts after goals (this is considred unsporting behavior by FIFA) and came into effect in 2004. This rule is strongly disliked by most players and fans in the more popular leagues and there is considerable pressure to either relax the rule remove it although this is unlikely so long as Blatter is President of FIFA. Some domestic associations, such as the English Football Association have mooted openly defying FIFA's directive and instructing their referees to not caution players for this particular celebration unless other circumstances warrant it.

Whilst a player who has been cautioned may continue to play in a game, many football jurisdictions have off-field penalties for players who accumulate a certain number of cautions in a season, tournament or phase of a tournament. Typically these take the form of a suspension from playing in their team's next game after that number of cautions has been reached. Such off-field penalties are determined by the competition's rules, and not by the Laws of the Game.

Yellow and red cards were first introduced in the game by British referee Ken Aston and their first major use was in the 1970 World Cup. The system of cautioning and sending-off existed prior to 1970; however, the use of coloured cards allows referees to convey their intentions directly regardless of the language spoken. The cards were only made mandatory at all levels in 1992.

Yellow cards are also in use in other sports, such as volleyball, women's lacrosse, field hockey, rugby union, rugby league in many countries, and handball. In both rugby codes, a player shown a yellow card is suspended from the game for 10 minutes (colloquially termed being sent to the sin bin). In the sport of fencing, a yellow card represents a warning, nullifying the action just made by a fencer. The Mixed Martial Arts promotion PRIDE also uses yellow cards, but a fighter can be shown two yellow cards without being disqualified, not one.

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• Red card