The problem of resolving the identity of a person can be categorized into two fundamentally distinct types of problems with different inherent complexities [1]: (i) verification and (ii) recognition. Verification (authentication) refers to the problem of confirming or denying a person's claimed identity (Am I who I claim I am?). Recognition (Who am I?) refers to the problem of establishing a subject's identity. A reliable personal identification is critical in many daily transactions. For example, access control to physical facilities and computer privileges are becoming increasingly important to prevent their abuse. There is an increasing interest in inexpensive and reliable personal identification in many emerging civilian, commercial, and financial applications. Typically, a person could be identified based on (i) a person's possession ("something that you possess"), e.g., permit physical access to a building to all persons whose identity could be authenticated by possession of a key; (ii) person's knowledge of a piece of information ("something that you know"), e.g., permit login access to a system to a person who knows the userid and a password associated with it. Another approach to positive identification is based on identifying physical characteristics of the person. The characteristics could be either a person's physiological traits, e.g., fingerprints, hand geometry, etc. or her behavioral characteristics, e.g., voice and signature. This method of identification of a person based on his/her physiological/behavioral characteristics is called biometrics. Since the biological characteristics can not be forgotten (like passwords) and can not be easily shared or misplaced (like keys), they are generally considered to be a more reliable approach to solving the personal identification problem.