Extragalactic background light is the faint diffuse light of the night sky that comes from distant but unresolved sources outside the Milky Way. EBL at optical, ultraviolet, and near-infrared (less than about 5 microns) wavelengths is thought to consist mainly of redshifted starlight from unresolved galaxies, with possible additional contributions from stars or gas in intergalactic space, and from decaying elementary particles. In the mid- and far-infrared, the main contribution is thought to be redshifted emission from dust particles, heated by starlight in galaxies. Studies of the EBL spectrum can serve as important tracers of the history of the formation of stars and galaxies.