In physics, the principle of locality states that an object is influenced directly only by its immediate surroundings. A theory that includes the principle of locality is said to be a "local theory". This is an alternative to the concept of instantaneous, or "non-local" action at a distance. Locality evolved out of the field theories of classical physics. The idea is that for a cause at one point to have an effect at another point, something in the space between those points must mediate the action. To exert an influence, something, such as a wave or particle, must travel through the space between the two points, carrying the influence.

The special theory of relativity limits the speed at which any such influence can travel to the speed of light, ${\displaystyle c}$. Therefore, the principle of locality implies that an event at one point cannot cause a simultaneous result at another point. An event at point ${\displaystyle A}$ cannot cause a result at point ${\displaystyle B}$ in a time less than ${\displaystyle T=D/c}$, where ${\displaystyle D}$ is the distance between the points and ${\displaystyle c}$ is the speed of light in vacuum.

Bell test experiments show that quantum mechanics broadly violates the inequalities established in Bell's theorem. According to some interpretations of quantum mechanics, this result implies that some quantum effects must be non-local.