The basic theory of card counting is to try to determine the likelihood of the next card you will be deal being the one you want.

As a very simple example, consider a game of Blackjack being played with only one deck of 52 cards. When the game begins, the chance of the first card being an ace is 4 in 52, or one in 13.

Casinos however have two advantages that stop such a simplistic method of card counting from being effective; they use multiple decks and have a stop point part of the way through the shoe so that they never deal all the cards from a shoe.

Nonetheless, a simple card counting strategy can be followed effectively using a "Plus or Minus" system.

In the Plus or Minus system a value is assigned to each card of +1, zero or -1 as follows:

Cards	Assigned Value
2, 3, 4, 5, 6	+1
7, 8, 9	0
10, Jack, Queen, King, Ace	-1

As each card is dealt on the table the player keeps a running count, adding 1 if the card is a 2, 3, 4, 5 or 6, and subtracting 1 for each 10-value card or Ace dealt.

When the running count is a positive figure the player has an advantage and bets high. When the running count is a negative figure they bet low.

Casinos do not like people card counting, however it is usually very difficult to pick up. An occasional player who uses this method and wins modest sums is unlikely to be noticed.

Casino staff will always become suspicious of anyone who bets large sums on one hand and small sums on the next and appears to be winning with any degree of consistency.

Although card counting is not illegal, if a casino suspects a player of card counting they may ban them. A casino does not actually need any proof of any misconduct. They are privately owned premises and have the right to ban anyone they wish without needing to give an explanation. If you attempt to enter a casino you have been banned from, this qualifies as trespassing.

In general however, casinos around the world expel and ban more than a thousand patrons for disorderly behavior for every one they ban for card counting.