A comprehensive theory that would relate the electromagnetic, gravitational, strong, and weak interactions (see fundamental interactions) in one set of equations. In its original context the expression referred only to the unification of general relativity and classical electromagnetic theory. No such theory has yet been found but some progress has been made in the unification of the electromagnetic and weak interactions (see electroweak theory).

Einstein attempted to derive quantum mechanics from unified-field theory, but it is now thought that any unified-field theory has to start with quantum mechanics. Attempts to construct unified-field theories, such as supergravity and Kaluza–Klein theory, have run into great difficulties. At the present time it appears that the framework of relativistic quantum field theory is not adequate to give a unified theory for all the known fundamental interactions and elementary particles, and that one has to go to extended objects, such as superstrings. Unified-field theories and other fundamental theories, such as superstring theory, are of great importance in understanding cosmology, particularly the early universe. In turn cosmology puts constraints on unified-field theories. See also grand unified theory.