For Discover, Visa and MasterCard the Security Code appears on the back-side of the credit card on the line where your signature is requested. Different credit issuers use different formats - some print all of (16) digits that comprise the credit card number, some include just the last (4) digits - all cards include (3) additional digits, these (3) digits are your Security Code.

Example: 5678 123 (the '123' is the security code)
Example: 4356 1234 5678 5678 123 (the '123' is the security code)
Example: 123 (the '123' is the security code)

For American Express cards the Security Code is located on the front of the card. The code appears as (4) digits printed above the last few raised/imprinted numbers that comprise the American Express Number.