Attempts to unify three fundamental interactions—strong, electromagnetic, and weak—with a postulate that the three forces, with the exception of gravity, can be unified into one at some very high energy. The basic idea is motivated by the incompleteness of the electroweak theory of S. Weinberg, A. Salam, and S. Glashow, which has been extremely successful in the energy region presently accessible with the use of accelerators, and by the observation that the coupling constant for strong nuclear forces becomes smaller as energy increases whereas the fine-structure constant (α = 1/137) for electromagnetic interactions is expected to increase with energy. The Weinberg-Salam-Glashow theory, which unifies electromagnetic and weak interactions, is incomplete in that strong interactions are not included; two coupling constants in the theory are unrelated; and in spite of many properties shared by them, quarks and leptons are unrelated, and their mass spectra and other properties remain unexplained.