During the course of a blackjack shoe, the dealer progressively exposes cards which are dealt to his or her own and the players' hands. Careful accounting of the exposed cards allows a player to make inferences about the cards which remain to be dealt, and use these inferences in one of two ways:
- The players can make larger bets when they have the advantage. For example, the players can increase the starting bet if there are many aces and tens left in the deck, in the hope of hitting a blackjack
- The players can deviate from basic strategy according to the composition of their undealt cards. For example, with many tens left in the deck, the players may double down in more situations since there is a better chance of getting a good hand.
A typical card counting system assigns a point score to each rank of card (e.g., 1 point for 2–6, 0 points for 7–9 and −1 point for 10–A). Whenever a card is exposed, a counter adds the score of that card to a running total, the 'count'; the count is used to make betting and playing decisions according to a table which they have learned. The count starts at 0 for a freshly shuffled deck for "balanced" counting systems. Unbalanced counts are often started at a value which depends on the number of decks used in the game.
In comparison to the house edge of 0.5–1% that can be achieved by playing basic strategy in most blackjack games, card counting can give the player an advantage of up to 2% over the house.
Card counting is most rewarding near the end of a complete shoe when as few as possible cards remain. Single-deck games are therefore particularly susceptible to card counting. As a result, casinos are more likely to insist that players do not reveal their cards to one another in single-deck games. In games with more decks of cards, casinos limit penetration by ending the shoe and initiating a reshuffle when one or more decks remain undealt, or by using a shuffling machine to reintroduce the exhausted cards every time a deck has been played.
Card counting mentally is legal and is not considered cheating. However, it usually needs to be done discreetly; if a player is detected while counting, the casino may inform them that they are no longer welcome to play blackjack there, or they may be completely banned from the property.
The use of any external devices to assist with counting cards is illegal in all US states that license blackjack card games.