Thermophotovoltaics (TPV) was intensively investigated as a technology for heat/electricity co-generation in the last decade of the 20th century. However, a wide-spread commercialisation has not been achieved yet. The world-record system efficiency for a TPV system using silicon photocells of 4% at 50W electrical output power as well as a maximum electrical output power of 164W, however at a lower efficiency of 0.84% could be demonstrated by a prototype system operating with an Yb2O3 selective emitter. Related developments of TPV system components such as radiation emitters, filters and photocells are reviewed and theoretical system simulations are compared to experimentally achieved results regarding system efficiency and the electrical output power. Finally, novel TPV applications are suggested and the commercial potential of this technology is discussed.