Where is my Card ID? On Visa, MasterCard or Discover cards, please turn your card over and look in the signature strip. You will find (either the entire 16-digit string of your card number, OR just the last 4 digits), followed by a space, followed by a 3-digit number. That 3-digit number is your Card ID number.   On American Express Cards, the Card ID number is a 4-digit number that appears above the end of your card number. The Card ID number is on the front of the credit card. The Card ID is used to verify that you have possession of the credit card you are attempting to use. The Card ID is used for your protection to prevent fraudulent activities on the Internet.