This is a process in which a player takes into account the cards that have already been removed from the shoe when determining their strategy. The most common adjustment made is to bet more money when the shoe is rich in cards that are good for the player, and bet less (or stop playing) when the remaining cards tilt the odds in favor of the house. However, card counters may also make adjustments to when they take insurance, when they surrender, or even to basic strategy plays based on the remaining cards in the shoe. These adjustments can leave the player with an advantage over the house.
Card counting is not illegal, but casinos may refuse to allow known card counters from playing blackjack. Card counting cannot be used in games that continuously shuffle the cards after each hand, such as in most online blackjack games.