Let's get the biggest misconception out of the way immediately: card counting is not illegal. Not in the United States, not in the UK, not in Canada, not in Australia. There is no federal or state law anywhere in these countries that makes it a crime to count cards at a blackjack table.
The reason is simple: card counting uses only your brain. You're not manipulating the cards, marking them, using a hidden computer, or colluding with the dealer. You're simply keeping a mental tally of which cards have been played, and that's not cheating. Courts have consistently upheld this position.
The landmark case is Ken Uston v. Resorts International Hotel (1982), where the New Jersey Supreme Court ruled that casinos cannot ban skilled players from table games. However, this ruling only applies in New Jersey. In every other state, casinos are private property and can refuse service to anyone for any reason, including being too good at blackjack.
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So what actually happens if a casino catches you counting? They won't call the police (unless you're using a device). Instead, they'll use one or more countermeasures: they'll ask you to flat bet (bet the same amount every hand), they'll shuffle more frequently to reset the count, they'll reduce deck penetration, or they'll simply 'back you off,' politely asking you to stop playing blackjack while offering you any other game in the casino.
If you refuse to stop or return after being backed off, you'll be formally trespassed. This means if you return, you CAN be arrested, not for counting cards, but for trespassing on private property. Casinos also share information through databases like the Griffin Book and OSN (Oregon Surveillance Network), so getting backed off at one casino can follow you to others.
The most famous card counting team in history, the MIT Blackjack Team, operated from 1979 to the early 2000s and won an estimated $5-10 million. They used a team approach: 'spotters' played minimum bets while keeping the count, then signaled 'big players' to join the table when the count was favorable. This made it harder for casinos to detect individual counters.
Modern casinos use sophisticated surveillance technology, including facial recognition software, betting pattern analysis algorithms, and trained pit bosses who watch for the telltale signs of counting: bet spreading (varying bets dramatically based on the count), deviating from basic strategy at specific counts, and playing for exactly as long as the shoe is favorable.
The house edge in blackjack with basic strategy is about 0.5%. A skilled counter using the Hi-Lo system can swing this to a 0.5-1.5% player edge when conditions are favorable. However, this edge is slim, the variance is enormous, and the risk of being banned is real. Card counting is a legitimate strategy, but it's far from the easy money Hollywood portrays.
Bottom line: card counting is legal, profitable when done correctly, but comes with real-world consequences. Casinos have every right to refuse your play (except in New Jersey), and they will exercise that right if they identify you as a counter. It's a cat-and-mouse game between skilled players and casino surveillance, and the casino usually has the advantage in that game too.