In physics, Fermi's interaction is an old explanation of the weak force, proposed by Enrico Fermi. Four fermions directly interact with one another. For example, this interaction is directly able to split a neutron (or a down-quark) to an electron, antineutrino and a proton (or an up-quark).

Tree Feynman diagrams describe the interaction remarkably well. Unfortunately, loop diagrams cannot be calculated reliably because Fermi's interaction is not renormalizable. The solution is to replace the four-fermion contact interaction by a more complete theory - an exchange of a W boson or a Z boson as explained in the electroweak theory. The electroweak theory is renormalizable.

Before the electroweak theory and the Standard Model were constructed, Richard Feynman and Murray Gell-Mann were able to determine the correct tensor structure (vector minus axial vector, V-A) of the four-fermion interaction.