Card counting is possible but extremely sensitive to the precise rules of the game being played. The principle is that you bet (more) when the cards remaining

Card counting is possible but extremely sensitive to the precise rules of the game being played. The principle is that you bet (more) when the cards remaining in the deck are favourable to you. Even if you can identify when that happens, precisely or heuristically, it's then also important to consider what percentage of your bank you can bet to best avoid bankruptcy. If you've considered all of this carefully enough, your method will likely no longer work when the casino modifies the rules in any way - most crudely by increasing their take to cover any variance in bet value. Edward O Thorp explains the mathematics of card counting extremely clearly and readably.