Thanks to Hollywood, everyone has an idea about counting cards at the casino. It's a technique in blackjack that can bring the odds into your favor by a tiny percentage. But enough to make a difference over the long term. The payer needs to keep a real and a running count in his head at the same time. From this, he'll know the type of cards left in the deck. Though counting cards is not illegal, and you'll not get arrested, as you can imagine, the casinos are not enamored with card counters.