As postulated in 1967 by Andrei Sakharov (1921–89), three conditions that are needed to explain why there is matter–antimatter asymmetry in the universe, rather than there being equal amounts of matter and antimatter. These conditions are: (1) there must be some process that violates baryon number or lepton number conservation; (2) there must be violation of charge conjugation and CP invariance; and (3) the universe is not in thermal equilibrium. A great deal of interest in Sakharov’s conditions was stimulated in the late 1970s when a number of authors suggested that the conditions could be satisfied in the context of grand unified theories (GUTs). Because GUTs in their simplest forms are not valid, there has been a great deal of activity aimed at finding which realistic theories Sakharov’s conditions would apply in.