Recent research has demonstrated distinct advantages of using thermal infrared imaging for improving face recognition performance. While conventional video cameras sense reflected light, thermal infrared cameras primarily measure emitted radiation from objects such as faces. Visible and thermal infrared image data collections of frontal faces have been on-going at NIST for over two years, producing the most comprehensive face database known to involve thermal infrared imagery. Rigorous experimentation with this database has revealed consistently superior recognition performance of algorithms when applied to thermal infrared, particularly under variable illumination conditions. Physical phenomenology responsible for this observation is analyzed. An end-to-end face recognition system incorporating simultaneous coregistered thermal infrared and visible has been developed and tested indoors with good performance.