Abstract: During the mergers of neutron star-neutron star and neutron star-black hole binaries, about several thousandths to hundredths of solar masses can be ejected outward due to the effects of tidal centrifugation, collision squeeze, and accretion feedback. This ejected material can be heated to a high temperature to produce rapidly evolving thermal emission in the ultravioletoptical-infrared bands, which is called kilonova emission. The emission timescale of a kilonova is from a few days to a few weeks, depending on the ejecta masses and opacities. The luminosity of a kilonova can reach at least about 10<sup<41</sup< erg s<sup<-1</sup<, which is specifically determined by the property of the heating sources including the radioactive decays of r-process nuclei and the possible energy release from the merger products. The kilonova phenomena was first discovered and confirmed in the GW170817 gravitational wave event.