While traditional access control systems might prompt a user to provide a proximity card or a smart card to gain access to a protected resource, a biometric security system requires the user to present their fingerprint (or other biometric) to the system, which is then matched against a stored image to verify the user's identity. Biometric access control systems tie a unique physical attribute of a user to the data that they are allowed to access. The most common biometric access systems use fingerprints, but these systems can also use iris and retinal scans, hand geometry, and facial recognition technology. Biometric technology represents a significant security advancement because it physically proves an individuals identity.