Card marking is the process of altering playing cards in a method only apparent to marker or conspirator, such as by bending or adding visible marks to a card. This allows different methods for card sharps to cheat or may be used for magic tricks. To be effective, the distinguishing mark or marks must be visible on the obverse sides of the cards, which are normally uniform. Card marking is often used to cheat when gambling or card tricks. Many casinos, particularly those in Las Vegas alter the decks of cards they sell to tourists – either by punching holes through the middle of cards or trimming their edges – to prevent cheaters from returning to the game tables after buying the cards and then slipping the favorable cards into their hands when playing.

Marked cards can be used regardless of who shuffles and deals the cards. Some more sophisticated marked-cards scams involve additional manipulative skills to steer the cards into the correct positions once the desired cards have been identified.

Some marked cards can be identified by performing the gamblers' riffle test (also known as "going to the movies"). Looking at the back of the cards while they are being riffled, the marks will "dance" around the back of the cards like an old-fashioned cartoon, provided that the viewer knows what sort of marks to look for. The riffle test is less effective for detecting cards marked with luminous and juice methods.