Tidal currents are formed by the energy dissipated by the tides. These can be thalassic, estuarine or fluvial. There has been a growing interest to harness these currents and turn the energy into electrical power. The potential is considerable. The technology is in existence or adaptable. The sites are numerous, though as with most ocean energies, only a small fraction of the global potential could be converted. The paper provides a brief review of an area that has been scrutinized for more than quarter of a century.