Is card counting in Blackjack legal?

Card counting is a mathematical strategy which is used in blackjack games to determine what cards have been already dealt from a deck.

Basics

The principle behind card counting is that high cards, particularly tens and aces are better for the player, while low cards, especially the 4s, 5s and 6s are better for the dealer. In contrast to the widely spread myth, card counters do not need special mental abilities in order to count cards, because they don't track and memorize specific cards. Card counters assign a point score to each card they see that matches the value of the card, and then only the sum is tracked. The myth that card counters keep track of all cards is the result of the film Rain Man.

Devices

Several types of card counting devices exist, but they are illegal in most American casinos.

Countermeasures

Casinos have spent a lot of money and energy in trying to thwart card counters. Such a measure is, for example, that casino staff harass card counters. This can be as simple as engaging the card counter in a talk in order to disturb his concentration. In some states it is possible to simply ban the card counters from the casino, using photos and facial recognition systems.