All credit and debit cards carry a security code number. This number is known to the bank and printed on the card, but is not stored or printed anywhere else. Therefore, it can be used as a check that when you make your purchase you are in physical possession of the card, or have at least seen the card at some time. On most cards, the security code number is the last three digits of the number printed on the back, at the top right of the signature strip. You will be required to enter this additional security number before your transaction can be completed.
*American Express puts the number on the front of the card above the credit card account number.
MASTERCARD, VISA:the last 3 digits of the number printed on the back of your card
AMERICAN EXPRESS:the 4 digits printed on the front of the card, on the right side.