Card counting is a strategy used in the game of blackjack to determine whether the next hand is likely to give an advantage to the player or the dealer. Players keep track of the ratio of high cards to low cards remaining in the deck. When there are more high cards left, the player has a better chance of winning. This technique involves assigning values to cards and maintaining a running count as cards are dealt. While card counting is not illegal, many casinos discourage it and may ask players to leave if they suspect counting is taking place.