Thermophotovoltaics (TPVs) are solid state devices that convert thermally generated radiation into electrical current. Any TPV device consists of a thermal emitter and a photovoltaic cell separated by a gap. TPVs were conceived in the 1950s but suffered from very low efficiency. The advent of low band-gap photovoltaics in the 1990's renewed interest in TPVs in hope of increasing their efficiency. In the 2000s, near-field radiation effects have been explored to further increase efficiency and power output. Much of the research on near-field effects has been focused on surface phonon polaritons since they can be excited thermally and can transfer energy in a narrow spectral band. The radiation transfer between (bulk) surfaces, thin films, photonic crystals, and metamaterials have been analyzed. The research seems to be headed towards analyzing arbitrary geometries and more realistic modeling.