Card counting is a strategy used in blackjack to track the cards that have been dealt, helping players determine whether the remaining deck is favorable to them or the dealer. By mentally keeping a "running count" of high and low cards, players adjust their bets and decisions accordingly. The technique can be done without any devices, relying solely on the player's ability to remember and calculate the count in real time. Although legal, casinos watch for card counters and may ask them to leave if they are suspected of using the method.