In Blackjack, insurance is perhaps the least understood of all the commonly available rules for Blackjack. This is not necessarily a bad thing because the insurance bet is normally a poor bet for the player, with a high house advantage. However, that's not always the case. So, here we go:

If the dealer turns an up-card of an Ace, he will offer "Insurance" to the players. Insurance bets can be made by betting up to half your original bet amount in the insurance betting stripe in front of your bet. The dealer will check to see if he has a 10-value card underneath his Ace, and if he does have Blackjack, your winning Insurance bet will be paid at odds of 2:1. You'll lose your original bet of course (unless you also have a Blackjack), so the net effect is that you break even (assuming you bet the full half bet for insurance.) This is why the bet is described as "insurance", since it seems to protect your original bet against a dealer Blackjack. Of course, if the dealer does not have Blackjack, you'll lose the insurance bet, and still have to play the original bet out.

In the simplest description, Insurance is a side-bet, where you are offered 2:1 odds that the dealer has a 10-valued card underneath ("in the hole"). A quick check of the odds yields this: In a single deck game, there are 16 ten-valued cards. Assuming that you don't see any other cards, including your own, the tens compose 16 out of 51 remaining cards after the dealer's Ace was removed. For the insurance bet to be a break-even bet, the hole card would have to be a ten 1 out of 3 times, but 16/51 is only 1 in 3.1875.

The situation is often thought to be different when you have a Blackjack. The dealer is likely to offer you "even money" instead of the insurance bet. This is just the same old insurance bet with a simplification thrown in. Let's ignore the "even money" name, and look at what happens when you insure a Blackjack. Let's say you bet $10, and have a Blackjack. You would normally collect $15 for this, unless the dealer also has a Blackjack, in which case you push or tie.

Let's assume that the dealer has an Ace up, and you decide to take insurance for the full amount, or $5. Now, two things can happen:

1) The dealer has a Blackjack. I tie with the $10, but collect 2:1 on the $5 insurance bet for a total profit of $10.
2) The dealer does not have Blackjack. I lose the $5, but collect $15 for my Blackjack. Total profit, again $10.
In either case, once I make the insurance bet, I'm guaranteed a profit of $10, or even money for my original bet.

So, casinos allow me to eliminate the insurance bet altogether, and simply declare that I want even money for my Blackjack when the dealer has an Ace showing. The Blackjack strategy player should simply never take the insurance bet, even the "even money" variety. Card counters on the other hand can often detect situations where more than one-third of the remaining cards are ten-valued, and the bet is then a profitable one. So, unless you know the bet is favorable, just ignore it.