Casinos have spent a great amount of effort and money in trying to thwart card counters. Countermeasures used to prevent card counters from profiting at blackjack include:
- Harassment of suspected card counters by casino staff. This may be as simple as engaging a suspected card counter in a conversation to break their concentration.
- Decreasing penetration, the percentage of the cards dealt before a shuffle. This reduces the ability of a counter to take advantage of a high count that has developed.
- Card-counter identification, using books of photos and new facial-recognition technologies to "blacklist" known counters.
- Computerized scanners in blackjack tables that can identify counting systems when in use.
- Heuristic systems that keep a count and track players' bets, looking for increases/decreases matching rises and falls in the count.
- Computer systems used in surveillance rooms that surveillance staff use to target suspect players to quantify their threat to the house.
- Shuffling when a player increases their wager.
- Changing rules for splitting, doubling down, or playing multiple hands. This also includes changing a table's stakes.
- Flat betting a player or making it so they cannot change the amount they bet during a shoe.