Biometrics is often refered to as "three factor" authentication, referring to the objective of establishing not only what you know and what you have, but also (or instead) what you are.

Biometric technologies at a glance

NEEDS MORE WORK / CROSS REFERENCES

CURRENT BIOMETRICS

Fingerprint

Iris scan

Hand scan

Voice print

OBSOLETE BIOMETRICS

Retina Scan

Signature Dynamics

NEW BIOMETRICS

DNA

Gait

Hybrid / Fusion methods

Relationship with other trust technologies

Biometrics PKI Note

"Authentication can be performed with three approaches: knowledge factor (something
the individual knows), possession factor (something the individual has), or
biometric factor (something physiologically unique about the individual). Biometric
technology uses a measurable biological or behavioral characteristic to reliably
distinguish one person from another. Biometrics can enhance PKI technology, and,
symbiotically, PKI technology can secure biometric technology. These PKI Notes
discuss the basics of biometric technology and its synergistic combination with PKI
technology".

Will Biometrics Obsolete PKI? A Special Report June 2001 American Bar Association, Bulletin of Law/Science & Technology.

This short paper discusses unique properties of PKI not provided by biometrics, including the ability to revoke when compromised, persistent signatures, and the ability to build open authentication systems.