Thermophotovoltaic technology has re-emerged during the last 8 years as being of potential importance for a wide variety of applications in both the military and non-military areas. The NREL TPV Conferences (of which there have been four) attract a diversity of scientists and engineers from all aspects of the technology, from the semiconductor converters to complete systems incorporating heat recuperation and photon recirculation. This book contains virtually all the papers presented at the 4th NREL TPV Conference: the presenters all being specialists in the field. This is the only source of complete state-of-the-art information on the subject. It also contains reviews of the seven individual sessions, prepared by the Session Chairs.