Mr Blackjack covers the legalities behind card counting and what the casinos will do if they catch you in the act.
Card counting is a skill, and therefore not illegal. Counting is not an offense under an state or federal law in the US, so casinos cannot convict or arrest you.
When combined with basic strategy and deviations, card counting gives players an edge in blackjack. Casinos lose money if these players count cards, so they will set limits or use techniques to block players from counting, like shuffling the deck more frequently. Banning players is generally a last resort for casinos.
Card counting is legal as long as you don’t use any aids to assist you. This is considered cheating and is against the law. Most stories around card counting focus on whether counters cheated by getting assistance from other players or apps.
Casinos are private premises. While they must follow the law, they can decide who to allow or ban from their grounds. Most casinos will try to limit or prevent you from card counting in blackjack, and then have a conversation, before asking you to leave.
Using a strategy chart requires some mathematical and gaming skill on the player’s part. Plus, the casino still has an edge in the game. Counting aids, like players peeking at the dealer’s cards, require no skill and give players an overt advantage.
There have been many card counters over the years. Edward Thorpe invented card counting, but the technique was popularized by the film 21, which told the story of the MIT blackjack team that won hundreds of thousands of dollars from Vegas casinos.
Many card counters will try to throw the casinos off their trail by making occasional basic strategy errors, or deviate away from their betting system.