Thermophotovoltaics (TPV) is a technique to convert radiation from a synthetic emitter into electricity by the use of photocells. In TPV there is a larger flexibility in achieving a high efficiency compared with solar photovoltaics (PV), where the solar spectrum as well as the radiation power is naturally fixed when no concentrating system is used. The emitter spectrum, however, can be widely varied by the choice of emitter material, its surface structure and its temperature. The radiation density on the cell surface depends on the distance to the emitter, as well as on the emission characteristics. In solar PV, the efficiency is increased by improving the photocell, especially by reducing the thermalization losses, which occur due to the absorption of the broad-band solar spectrum. Thirdgeneration PV photocell designs often use a band structure with several transitions to adapt the cell to the Sun’s broad-band spectrum. In TPV the system’s efficiency is optimized by spectrally matching the emitter to the photocells. By using conventional single bandgap photocells, a high efficiency is achieved for radiation from a selective emitter with a narrow-band radiation spectrum located closely above the bandgap of the cells.