In physics, the graviton is a hypothetical elementary particle that transmits the force of gravity in some quantum gravity theories. If it exists, the graviton must be massless (because the gravitational force has unlimited range) and must have a spin of 2 (because gravity is a second-rank tensor field).

There is no hope of detecting gravitons in the foreseeable future^{[citation needed]}. They are postulated because of the success of the Standard Model at modelling the behavior of all other forces of nature with similar particles: electromagnetism with the photon, the strong interaction with the gluons, and the weak interaction with the W and Z bosons. However, naive attempts to extend the Standard Model with gravitons run into serious theoretical difficulties. The analogy which motivates the introduction of gravitons is controversial, because of gravitation's special role (in general relativity) of defining the spacetime in which other events take place. The Standard Model requires a fixed background spacetime.