Visa/Mastercard and Discover Users
Flip your card over and look at the signature box. You should see either the entire 16-digit credit card number or just the last four digits followed by a special 3-digit code. This 3-digit code is your Card Security Code.
 

American Express Credit Card Users
Look for the 4-digit code printed on the front of your card just above and to the right of your main credit card number. This 4-digit code is your Card Identification Number (CID). The CID is the four-digit code printed just above the Account Number.