On MasterCard, the Security Code is the 3-digit code located on the back of your card. On Visa cards, the Security Code is the 3-digit code located on the back of your card. On American Express cards, the Security Code is the 4-digit code located on the front of your card. On Discover cards, the Security Code is the 3-digit code located on the back of your card. The Security Code is the 3 to 4 digit code located on the back/front of your card that is used to verification purposes.