Card counting is in some ways the Holy Grail of the casino and gambling world: a genuine system that gives you a true edge and tips the odds in your favor, meaning that in the long term the player, not the casino, has the advantage.
Card counting is possible in blackjack unlike most casino games, roulette being the most obvious example, what has happened previously in roulette has no bearing on what will happen next. If red comes up 10 times in a row on roulette, the chances of the next spin being black (or red, or zero for that matter) remain the same as they always were.
However, in blackjack, previous cards do have an impact on what is to come, or at least that was certainly true in bricks and mortar casinos that used a traditional eight decks that were shuffled manually.
Card counting revolves around the central point that a pack stacked with high cards (10s and aces) is beneficial to the player, because it gives them the best chance of hitting blackjack whilst making it more likely the dealer will bust (as they have to hit on less than 17).
Players keep a running count of the high and low cards that have already been dealt in order to assess whether the state of the remaining cards is favorable or not. By betting big when the deck is full of high cards and betting less when it is less favorable, card counters can overcome the small house edge that exists in blackjack and actually have a positive expectation from their play. In simple terms, the player not the casino wins in the long term.
Unfortunately casinos have taken a number of measures to combat card counting. Card counting is legal and while casinos can always refuse to allow certain players to play, it remains in their interests to make counting cards impossible or at least reduce its efficacy.
Casinos may use continuous shuffling machines (online and offline) to negate the benefits of card counting, while in real world casinos fast dealing, distracting players by talking to them and using software to spot card counters are all well known tactics. Equally shuffling the cards earlier into the deck reduces the benefits of card counting. So while card counting does work, the casinos certainly don’t make it easy for the counters!
Unfortunately card counting is not a realistic strategy when playing blackjack online. When you play blackjack online you have no idea what the software is doing to the cards. Are they shuffling them after every hand? Shuffling after every 10 hands? Using an 8 deck shoe? You have no way of knowing when playing online so we do not recommend trying to count cards when playing at a USA online casino. In fact many online casinos use a constant shuffle meaning the cards are in fact shuffled after every hand, making counting cards irrelevant.