Grand unification, Grand unified theory or GUT (a misnomer, really) refers to a theory in physics that unifies the strong interaction and electroweak interaction. Several such theories have been proposed, but none is currently universally accepted. The (future) theory that will also include gravity is termed theory of everything.

GUTs also predict the existence of topological defects such as monopoles, cosmic strings, domain walls, and others. None have been observed and their absence is known as the monopole problem in cosmology.

Some common GUT groups are:

• SU(5), Georgi-Glashow model
• SO(10)
• SU(5)XXU(1), Flipped SU(5).
• SU(4)XXSU(2)XXSU(2), Pati-Salam model
• SU(3)XXSU(3)XXSU(3), Trinification
• E₆
• Technicolor models

Note that it is meant that these groups are Lie algebras. The Lie group could be , just to take a random example.

As of 2003, there is still no hard evidence nature is described by a GUT theory. In fact, since the Higgs particle hasn't been discovered yet, it's not even certain if the Standard Model is fully accurate.