Abstract This paper reviews grand unified theories and some of their possible applications to cosmology. The philosophy of grand unification to be followed is first developed, some low-energy tests described, and then expectations for new interactions causing baryon decay and neutrino masses are presented. The experimental situations concerning these two possibilities are briefly reviewed. A discussion is given of the possible relevance of baryon-number violating reactions in grand unified theories to understanding the problem of baryosynthesis, and a possible connection with the neutron electric dipole moment is mentioned. Possible interfaces between cosmology and particle physics involving neutrinos are mentioned.